反转二叉树

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        root.left = invertTree(root.left);
        root.right = invertTree(root.right);
        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;
        return root;
    }
}

object-c实现：

递归方式：

/** 
 * 翻转二叉树（又叫：二叉树的镜像） 
 *
 * @param rootNode 根节点
 *
 * @return 翻转后的树根节点（其实就是原二叉树的根节点） 
 */
 + (BinaryTreeNode *)invertBinaryTree:(BinaryTreeNode *)rootNode {
    if (!rootNode) {  
        return nil; 
    } 
    if (!rootNode.leftNode & !rootNode.rightNode) {
        return rootNode; 
    } 
    [self invertBinaryTree:rootNode.leftNode];
    [self invertBinaryTree:rootNode.rightNode]; 
    BinaryTreeNode *tempNode = rootNode.leftNode; 
    rootNode.leftNode = rootNode.rightNode;
    rootNode.rightNode = tempNode; 
    return rootNode;
  }

非递归方式：

+ (BinaryTreeNode *)invertBinaryTree:(BinaryTreeNode *)rootNode {
    if (!rootNode) {  
        return nil; 
    }
    if (!rootNode.leftNode & !rootNode.rightNode) {  
        return rootNode; 
    }
    NSMutableArray *queueArray = [NSMutableArray array]; //数组当成队列
    [queueArray addObject:rootNode]; //压入根节点
    while (queueArray.count > 0) {
        BinaryTreeNode *node = [queueArray firstObject];
        [queueArray removeObjectAtIndex:0]; //弹出最前面的节点，仿照队列先进先出原则
        BinaryTreeNode *pLeft = node.leftNode;
        node.leftNode = node.rightNode;
        node.rightNode = pLeft;

        if (node.leftNode) {
            [queueArray addObject:node.leftNode];
        }
        if (node.rightNode) {
            [queueArray addObject:node.rightNode];
        }
    }
    return rootNode;
}