144. 二叉树的前序遍历
地址:https://leetcode-cn.com/problems/binary-tree-preorder-traversal/
94.二叉树的中序遍历
地址:https://leetcode-cn.com/problems/binary-tree-inorder-traversal/
145.二叉树的后序遍历
地址:https://leetcode-cn.com/problems/binary-tree-postorder-traversal/
102.二叉树的层序遍历
地址:https://leetcode-cn.com/problems/binary-tree-level-order-traversal/
给定一个二叉树的根节点 root ，返回它的 中序 遍历。
示例 1：
输入：root = [1,null,2,3]
输出：[1,3,2]
示例 2：
输入：root = []
输出：[]
示例 3：
输入：root = [1]
输出：[1]
示例 4：
输入：root = [1,2]
输出：[2,1]
示例 5：
输入：root = [1,null,2]
输出：[1,2]
提示：
树中节点数目在范围 [0, 100] 内
-100 <= Node.val <= 100
图片显示不出来，请直接查看力扣原地址
前序遍历 先遍历根，再访问左子树，再遍历右子树
中序遍历 先遍历左子树，再访问根，再遍历右子树
后序遍历 先遍历根，再访问右子树，再遍历左子树
使用OC 我们需要创建一个BinaryTreeNode节点model 里面的对象如下
/**
* 值
*/
@property (nonatomic, assign) NSInteger value;
/**
* 左节点
*/
@property (nonatomic, strong) BinaryTreeNode *leftNode;
/**
* 右节点
*/
@property (nonatomic, strong) BinaryTreeNode *rightNode;
方法1:递归
/**
* 前序遍历
* 先遍历根，再访问左子树，再遍历右子树
*
* @param rootNode 根节点
*/
- (NSArray *)preorderTraversal1:(BinaryTreeNode *)rootNode
{
if (rootNode == nil) {
return @[];
}
NSMutableArray *inorderArray = [NSMutableArray array];
[inorderArray addObject:@(rootNode.value)];
if (rootNode.leftNode) {
NSArray *leftArray = [self preorderTraversal1:rootNode.leftNode];
[inorderArray addObjectsFromArray:leftArray];
}
if (rootNode.rightNode) {
NSArray *rightArray = [self preorderTraversal1:rootNode.rightNode];
[inorderArray addObjectsFromArray:rightArray];
}
return inorderArray;
}
/**
* 中序遍历
* 先遍历左子树，再访问根，再遍历右子树
*
* @param rootNode 根节点
*/
- (NSArray *)inorderTraversal1:(BinaryTreeNode *)rootNode
{
if (rootNode == nil) {
return @[];
}
NSMutableArray *inorderArray = [NSMutableArray array];
if (rootNode.leftNode) {
NSArray *leftArray = [self inorderTraversal1:rootNode.leftNode];
[inorderArray addObjectsFromArray:leftArray];
}
[inorderArray addObject:@(rootNode.value)];
if (rootNode.rightNode) {
NSArray *rightArray = [self inorderTraversal1:rootNode.rightNode];
[inorderArray addObjectsFromArray:rightArray];
}
return inorderArray;
}
/**
* 后序遍历
* 先访问左子树，再遍历右子树 最后根
*
* @param rootNode 根节点
*/
- (NSArray *)postorderTraversal1:(BinaryTreeNode *)rootNode
{
if (rootNode == nil) {
return @[];
}
NSMutableArray *inorderArray = [NSMutableArray array];
if (rootNode.leftNode) {
NSArray *leftArray = [self postorderTraversal1:rootNode.leftNode];
[inorderArray addObjectsFromArray:leftArray];
}
if (rootNode.rightNode) {
NSArray *rightArray = [self postorderTraversal1:rootNode.rightNode];
[inorderArray addObjectsFromArray:rightArray];
}
[inorderArray addObject:@(rootNode.value)];
return inorderArray;
}
递归比较简单 没啥好说的
方法2:迭代
/// 前序遍历 迭代
/// @param rootNode 根节点
- (NSArray *)preorderTraversal2:(BinaryTreeNode *)rootNode
{
if (rootNode == nil) {
return @[];
}
NSMutableArray *inorderArray = [NSMutableArray array];
NSMutableArray *numbers = [NSMutableArray array];
while (rootNode != nil || inorderArray.count > 0) {
///将根节点的值取出来，放入数组中 将左节点设为根节点 知道左节点为空，这时根节点和左子树的值都放入数组中
while (rootNode != nil) {
[numbers addObject:@(rootNode.value)];
[inorderArray addObject:rootNode];
rootNode = rootNode.leftNode;
}
///取出数组的最上面的值，开始读取右子树
rootNode = [inorderArray lastObject];
[inorderArray removeLastObject];
rootNode = rootNode.rightNode;
}
return numbers;
}
/// 中序遍历 迭代
/// @param rootNode 根节点
- (NSArray *)inorderTraversal2:(BinaryTreeNode *)rootNode
{
if (rootNode == nil) {
return @[];
}
NSMutableArray *inorderArray = [NSMutableArray array];
NSMutableArray *numbers = [NSMutableArray array];
while (rootNode != nil || inorderArray.count > 0) {
//判断节点是否为空 不为空放入数组中 然后将左节点设为跟节点 再次便利
while (rootNode != nil) {
[inorderArray addObject:rootNode];
rootNode = rootNode.leftNode;
}
//最后加入的左节点 也就是最左的节点 这个节点没有左节点，下面该访问右节点
rootNode = [inorderArray lastObject];
[inorderArray removeLastObject];
[numbers addObject:@(rootNode.value)];
//将右节点作为跟节点 这里需要判断有无左节点
rootNode = rootNode.rightNode;
}
return numbers;
}
/// 后序遍历 迭代
/// @param rootNode 根节点
- (NSArray *)postorderTraversal2:(BinaryTreeNode *)rootNode
{
if (rootNode == nil) {
return @[];
}
NSMutableArray *inorderArray = [NSMutableArray array];
NSMutableArray *numbers = [NSMutableArray array];
while (rootNode != nil || inorderArray.count > 0) {
while (rootNode != nil) {
[numbers addObject:@(rootNode.value)];
[inorderArray addObject:rootNode];
rootNode = rootNode.rightNode;
}
rootNode = [inorderArray lastObject];
[inorderArray removeLastObject];
rootNode = rootNode.leftNode;
}
NSMutableArray *newNumbers = [NSMutableArray array];
for (NSInteger i = numbers.count-1; i>=0; i--) {
[newNumbers addObject:numbers[i]];
}
return newNumbers;
}
/// 层序遍历 迭代
/// @param rootNode 根节点
- (NSArray *)levelOrder:(BinaryTreeNode *)rootNode
{
if (rootNode == nil) {
return @[];
}
NSMutableArray *dataArray = [NSMutableArray array];
[dataArray addObject:rootNode];
NSMutableArray *numbers = [NSMutableArray array];
NSMutableArray *values = [NSMutableArray array];
NSInteger count = dataArray.count;
while (dataArray.count > 0) {
BinaryTreeNode *node = [dataArray firstObject];
[values addObject:@(node.value)];
count--;
[dataArray removeObjectAtIndex:0];
if (node.leftNode) {
[dataArray addObject:node.leftNode];
}
if (node.rightNode) {
[dataArray addObject:node.rightNode];
}
if (count == 0) {
count = dataArray.count;
[numbers addObject:values];
values = [NSMutableArray array];
}
}
return numbers;
}
inorderArray 这里表示队列 每次取出最后一个 后进先出
numbers 这里表示节点
重点是先将根节点 和 左节点 加入数组中，直到找到最左的节点
开始取出
取出后 在将右节点加入
验证
// 144. 二叉树的前序遍历 https://leetcode-cn.com/problems/binary-tree-preorder-traversal/
// 94. 二叉树的中序遍历 https://leetcode-cn.com/problems/binary-tree-inorder-traversal/
// 145. 二叉树的后序遍历 https://leetcode-cn.com/problems/binary-tree-postorder-traversal/
// 102. 二叉树的层序遍历 https://leetcode-cn.com/problems/binary-tree-level-order-traversal/
{
// 输入：root = [1,null,2,3]
// 前序遍历 输出：[1,2,3]
// 中序遍历 输出：[1,3,2]
// 后序遍历 输出：[3,2,1]
// 层次遍历 输出：[[1],[2],[3]]
BinaryTreeNode *node1 = [[BinaryTreeNode alloc] init];
node1.value = 1;
BinaryTreeNode *node2 = [[BinaryTreeNode alloc] init];
node2.value = 2;
BinaryTreeNode *node3 = [[BinaryTreeNode alloc] init];
node3.value = 3;
node1.rightNode = node2;
node2.leftNode = node3;
///前序遍历
NSArray *preorderArray1 = [self preorderTraversal1:node1];
NSLog(@"preorderArray1 is %@",preorderArray1);
NSArray *preorderArray2 = [self preorderTraversal2:node1];
NSLog(@"preorderArray2 is %@",preorderArray2);
///中序遍历
NSArray *inorderArray1 = [self inorderTraversal1:node1];
NSLog(@"inorderArray1 is %@",inorderArray1);
NSArray *inorderArray2 = [self inorderTraversal2:node1];
NSLog(@"inorderArray2 is %@",inorderArray2);
///后序遍历
NSArray *postorderArray1 = [self postorderTraversal1:node1];
NSLog(@"postorderArray1 is %@",postorderArray1);
NSArray *postorderArray2 = [self postorderTraversal2:node1];
NSLog(@"postorderArray2 is %@",postorderArray2);
///层次遍历
NSArray *levelOrderArray1 = [self levelOrder:node1];
NSLog(@"levelOrderArray1 is %@",levelOrderArray1);
}
{
// 输入：root = []
// 四大遍历 输出：[]
///前序遍历
NSArray *preorderArray1 = [self preorderTraversal1:nil];
NSLog(@"preorderArray1 is %@",preorderArray1);
NSArray *preorderArray2 = [self preorderTraversal2:nil];
NSLog(@"preorderArray2 is %@",preorderArray2);
///中序遍历
NSArray *inorderArray1 = [self inorderTraversal1:nil];
NSLog(@"inorderArray1 is %@",inorderArray1);
NSArray *inorderArray2 = [self inorderTraversal2:nil];
NSLog(@"inorderArray2 is %@",inorderArray2);
///后序遍历
NSArray *postorderArray1 = [self postorderTraversal1:nil];
NSLog(@"postorderArray1 is %@",postorderArray1);
NSArray *postorderArray2 = [self postorderTraversal2:nil];
NSLog(@"postorderArray2 is %@",postorderArray2);
///层次遍历
NSArray *levelOrderArray1 = [self levelOrder:nil];
NSLog(@"levelOrderArray1 is %@",levelOrderArray1);
}
{
// 输入：root = [1]
// 前序遍历、中序遍历、后序遍历 输出：[1]
// 层次遍历 输出：[[1]]
BinaryTreeNode *node1 = [[BinaryTreeNode alloc] init];
node1.value = 1;
///前序遍历
NSArray *preorderArray1 = [self preorderTraversal1:node1];
NSLog(@"preorderArray1 is %@",preorderArray1);
NSArray *preorderArray2 = [self preorderTraversal1:node1];
NSLog(@"preorderArray2 is %@",preorderArray2);
///中序遍历
NSArray *inorderArray1 = [self inorderTraversal1:node1];
NSLog(@"inorderArray1 is %@",inorderArray1);
NSArray *inorderArray2 = [self inorderTraversal2:node1];
NSLog(@"inorderArray2 is %@",inorderArray2);
///后序遍历
NSArray *postorderArray1 = [self postorderTraversal1:node1];
NSLog(@"postorderArray1 is %@",postorderArray1);
NSArray *postorderArray2 = [self postorderTraversal2:node1];
NSLog(@"postorderArray2 is %@",postorderArray2);
///层次遍历
NSArray *levelOrderArray1 = [self levelOrder:node1];
NSLog(@"levelOrderArray1 is %@",levelOrderArray1);
}
{
// 输入：root = [1,2]
// 前序遍历 输出：[1,2]
// 中序遍历 输出: [2,1]
// 后序遍历 输出：[2,1]
// 层次遍历 输出：[[1],[2]]
BinaryTreeNode *node1 = [[BinaryTreeNode alloc] init];
node1.value = 1;
BinaryTreeNode *node2 = [[BinaryTreeNode alloc] init];
node2.value = 2;
node1.leftNode = node2;
///前序遍历
NSArray *preorderArray1 = [self preorderTraversal1:node1];
NSLog(@"preorderArray1 is %@",preorderArray1);
NSArray *preorderArray2 = [self preorderTraversal1:node1];
NSLog(@"preorderArray2 is %@",preorderArray2);
///中序遍历
NSArray *inorderArray1 = [self inorderTraversal1:node1];
NSLog(@"inorderArray1 is %@",inorderArray1);
NSArray *inorderArray2 = [self inorderTraversal2:node1];
NSLog(@"inorderArray2 is %@",inorderArray2);
///后序遍历
NSArray *postorderArray1 = [self postorderTraversal1:node1];
NSLog(@"postorderArray1 is %@",postorderArray1);
NSArray *postorderArray2 = [self postorderTraversal2:node1];
NSLog(@"postorderArray2 is %@",postorderArray2);
///层次遍历
NSArray *levelOrderArray1 = [self levelOrder:node1];
NSLog(@"levelOrderArray1 is %@",levelOrderArray1);
}
{
// 输入：root = [1,null,2]
// 前序遍历、中序遍历 输出：[1,2]
// 后序遍历 输出：[2,1]
// 层次遍历 输出：[[1],[2]]
BinaryTreeNode *node1 = [[BinaryTreeNode alloc] init];
node1.value = 1;
BinaryTreeNode *node2 = [[BinaryTreeNode alloc] init];
node2.value = 2;
node1.rightNode = node2;
///前序遍历
NSArray *preorderArray1 = [self preorderTraversal1:node1];
NSLog(@"preorderArray1 is %@",preorderArray1);
NSArray *preorderArray2 = [self preorderTraversal1:node1];
NSLog(@"preorderArray2 is %@",preorderArray2);
///中序遍历
NSArray *inorderArray1 = [self inorderTraversal1:node1];
NSLog(@"inorderArray1 is %@",inorderArray1);
NSArray *inorderArray2 = [self inorderTraversal2:node1];
NSLog(@"inorderArray2 is %@",inorderArray2);
///后序遍历
NSArray *postorderArray1 = [self postorderTraversal1:node1];
NSLog(@"postorderArray1 is %@",postorderArray1);
NSArray *postorderArray2 = [self postorderTraversal2:node1];
NSLog(@"postorderArray2 is %@",postorderArray2);
///层次遍历
NSArray *levelOrderArray1 = [self levelOrder:node1];
NSLog(@"levelOrderArray1 is %@",levelOrderArray1);
}
{
// 输入：root = [3,9,20,null,null,15,7]
// 前序遍历 输出：[3,9,20,15,7]
// 中序遍历 输出：[9,3,15,20,7]
// 后序遍历 输出：[9,15,7,20,3]
// 层次遍历输出：[[3],[9,20],[15,7]]
BinaryTreeNode *node1 = [[BinaryTreeNode alloc] init];
node1.value = 3;
BinaryTreeNode *node2 = [[BinaryTreeNode alloc] init];
node2.value = 9;
BinaryTreeNode *node3 = [[BinaryTreeNode alloc] init];
node3.value = 20;
node1.leftNode = node2;
node1.rightNode = node3;
BinaryTreeNode *node4 = [[BinaryTreeNode alloc] init];
node4.value = 15;
BinaryTreeNode *node5 = [[BinaryTreeNode alloc] init];
node5.value = 7;
node3.leftNode = node4;
node3.rightNode = node5;
///前序遍历
NSArray *preorderArray1 = [self preorderTraversal1:node1];
NSLog(@"preorderArray1 is %@",preorderArray1);
NSArray *preorderArray2 = [self preorderTraversal1:node1];
NSLog(@"preorderArray2 is %@",preorderArray2);
///中序遍历
NSArray *inorderArray1 = [self inorderTraversal1:node1];
NSLog(@"inorderArray1 is %@",inorderArray1);
NSArray *inorderArray2 = [self inorderTraversal2:node1];
NSLog(@"inorderArray2 is %@",inorderArray2);
///后序遍历
NSArray *postorderArray1 = [self postorderTraversal1:node1];
NSLog(@"postorderArray1 is %@",postorderArray1);
NSArray *postorderArray2 = [self postorderTraversal2:node1];
NSLog(@"postorderArray2 is %@",postorderArray2);
///层次遍历
NSArray *levelOrderArray1 = [self levelOrder:node1];
NSLog(@"levelOrderArray1 is %@",levelOrderArray1);
}
{
BinaryTreeNode *node1 = [BinaryTreeManager createTreeWithValues:@[@6,@1,@2,@3,@4,@5,@7,@8,@9,@10,@11]];
///前序遍历
NSArray *preorderArray1 = [self preorderTraversal1:node1];
NSLog(@"preorderArray1 is %@",preorderArray1);
NSArray *preorderArray2 = [self preorderTraversal1:node1];
NSLog(@"preorderArray2 is %@",preorderArray2);
///中序遍历
NSArray *inorderArray1 = [self inorderTraversal1:node1];
NSLog(@"inorderArray1 is %@",inorderArray1);
NSArray *inorderArray2 = [self inorderTraversal2:node1];
NSLog(@"inorderArray2 is %@",inorderArray2);
///后序遍历
NSArray *postorderArray1 = [self postorderTraversal1:node1];
NSLog(@"postorderArray1 is %@",postorderArray1);
NSArray *postorderArray2 = [self postorderTraversal2:node1];
NSLog(@"postorderArray2 is %@",postorderArray2);
///层次遍历
NSArray *levelOrderArray1 = [self levelOrder:node1];
NSLog(@"levelOrderArray1 is %@",levelOrderArray1);
}