iOS实现反转二叉树（前序遍历二叉树）

题目:

Input:

Output:

首先分析下这个二叉树,从上往下看发现这个树是把树上的数据进行了交换,但是仔细一看发现最后一排的1-3反转过去后变成了3-1.所以得出结论,这道题是左右子树进行了交换,用函数递归就能很容易实现了.

## OC版本

声明节点属性：

#import 

NS_ASSUME_NONNULL_BEGIN

@interface TreeNodel : NSObject

- (instancetype)initWithVal:(NSInteger)val;

@property (strong, nonatomic) TreeNodel *left;
@property (strong, nonatomic) TreeNodel *right;
@property (assign, nonatomic) NSInteger val;


@end

NS_ASSUME_NONNULL_END

实现代码：

/// 反转二叉树
/// @param root 二叉树
- (TreeNodel *)invertTree:(TreeNodel *)root
{
    //递归调用有一个明确的出口
    if (!root) {
        return root;
    }
    //不满足条件时自己调用自己
    [self invertTree:root.left];
    [self invertTree:root.right];
    [self exchangeNode:root];
    return root;
}

- (void)exchangeNode:(TreeNodel *)node {
    if (node) {
        TreeNodel *temp = node.left;
        node.left = node.right;
        node.right = temp;
    }
}

测试：

- (void)viewDidLoad {
    [super viewDidLoad];
    
    TreeNodel *one = [[TreeNodel alloc]initWithVal:4];
    one.left = [[TreeNodel alloc]initWithVal:2];
    one.right = [[TreeNodel alloc]initWithVal:7];
    
    TreeNodel *two = one.left;
    two.left = [[TreeNodel alloc]initWithVal:1];
    two.right = [[TreeNodel alloc]initWithVal:3];

    TreeNodel *there = one.right;
    there.left = [[TreeNodel alloc]initWithVal:6];
    there.right = [[TreeNodel alloc]initWithVal:9];
    
    NSLog(@"层次遍历 = %@",[self leveold:one]);
    NSLog(@"反转之后层次遍历 = %@",[self leveold:[self invertTree:one]]);
    

}

结果：

后记：
前序遍历二叉树：

/// 二叉树的前序遍历
/// @param node 二叉树
- (NSArray *)preorderTravelsal:(TreeNodel *)node{
    
    NSMutableArray *res = [NSMutableArray array];
    NSMutableArray *stack = [NSMutableArray array];
    
    while (node != nil || stack.count > 0) {
        if (node != nil) {
            [res addObject:@(node.val)];
            [stack addObject:node];
            node = node.left;
        }else{
        
            TreeNodel *lastNode = stack.lastObject;
            node = lastNode.right;
            [stack removeLastObject];
        }
        
    }
    return res;
}

结果：