数据结构的学习历程06
数据结构的学习历程06
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- 二叉树部分习题集
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- 二叉树转字符串
- 字符串创建二叉树
- 最近公共祖先
- 把二叉搜素树转成有序双向链表
- 根据前与中序遍历构造二叉树
- 根据中后序遍历结果构建二叉树
- 非递归前序遍历
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二叉树部分习题集
二叉树转字符串
leetcode：二叉树转字符串
public String tree2str(TreeNode root) {
String str = "";
if(root == null) return str;
str = str+root.val;
if (root.right == null && root.left == null) {
return str;
}else if (root.right == null &&root.left != null) {
str = str + "("+ tree2str(root.left) + ")";
}else if (root.right != null && root.left == null){
str = str+"()";
str = str + "("+ tree2str(root.right) + ")";
}else if (root.right != null && root.left != null){
str = str + "("+ tree2str(root.left) + ")";
str = str + "("+ tree2str(root.right) + ")";
}
return str;
}
字符串创建二叉树
leetcode：创建二叉树
public class Main {
static int i=0;
public static Node creteTree(String str) {
Node root = null;
if (str.charAt(i)!= '#' ){
root = new Node(str.charAt(i));
i++;
root.left = creteTree(str);
root.right = creteTree(str);
}else {
i++;
}
return root;
}
public static void inorder(Node root){
if (root == null) return;
inorder(root.left);
System.out.print(root.val+" ");
inorder(root.right);
}
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
String str = sc.nextLine();
int i =0;
Node root = creteTree(str);
inorder(root);
}
}
最近公共祖先
LCA问题：最近公共祖先（lowestCommonAncestor）
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if (root == null) return root;
if (root == p || root == q) return root;
TreeNode left = lowestCommonAncestor(root.left,p,q);
TreeNode right = lowestCommonAncestor(root.right,p,q);
if (left != null && right != null) return root;
if (left == null && right != null) return right;
if (left != null && right == null) return left;
return null;
}
把二叉搜素树转成有序双向链表
牛客：二叉搜索树转双向链表
leetcode：二叉搜索树转累加树
二叉搜索树：
TreeNode cur = null;
public void Convert1(TreeNode pRootOfTree) {
if (pRootOfTree == null) return ;
Convert1(pRootOfTree.left);
pRootOfTree.left = cur;
if(cur != null){
cur.right = pRootOfTree;
}
cur = pRootOfTree;
Convert1(pRootOfTree.right);
}
public TreeNode Convert(TreeNode pRootOfTree) {
if (pRootOfTree == null) return null;
Convert1(pRootOfTree);
while (pRootOfTree.left != null){
pRootOfTree=pRootOfTree.left;
}
return pRootOfTree;
}
根据前与中序遍历构造二叉树
leetcode：根据前与中序遍历构造二叉树
int k = 0;
public TreeNode creatTree(int[] preorder,int[] inorder,int b,int e) {
if (b > e) return null;
TreeNode root = new TreeNode(preorder[k]);//3
int index = findk(inorder,preorder[k]);//1 0
k++;
root.left = creatTree(preorder,inorder,b,index-1);//0,0 2,2
root.right = creatTree(preorder,inorder,index+1,e);//2,e
return root;
}
public int findk(int[] inorder , int k){
int i = 0;
for (; i < inorder.length; i++) {
if (k == inorder[i])
break;
}
return i;
}
public TreeNode buildTree(int[] preorder, int[] inorder) {
return creatTree(preorder,inorder,0,preorder.length-1);
}
根据中后序遍历结果构建二叉树
leetcode:根据中后序遍历结果构建二叉树
int k = 0;
public TreeNode creatTree(int[] inorder,int[] postorder,int b,int e) {
if (b > e) return null;
TreeNode root = new TreeNode(postorder[k]);//3
int index = findk(inorder,postorder[k]);//1 0
k--;
root.right = creatTree(inorder,postorder,index+1,e);//2,e
root.left = creatTree(inorder,postorder,b,index-1);//0,0 2,2
return root;
}
public int findk(int[] inorder , int k){
int i = 0;
for (; i < inorder.length; i++) {
if (k == inorder[i])
break;
}
return i;
}
public TreeNode buildTree(int[] inorder, int[] postorder) {
k = inorder.length-1;
return creatTree(inorder,postorder,0,postorder.length-1);
}
非递归前序遍历
leetcode:非递归前序遍历
void preOrderTraversal(TreeNode root){
if (root == null) return;
Stack<TreeNode> sta = new Stack<>();
sta.push(root);
while (!sta.empty()){
TreeNode cur = sta.pop();
System.out.println(cur.val);
if (cur.right!=null){
sta.push(cur.right);
}
if (cur.left!=null){
sta.push(cur.left);
}
}
}