LeetCode 226. Invert Binary Tree

原题链接在这里：https://leetcode.com/problems/invert-binary-tree/

题目：

Invert a binary tree.

     4
   /   
  2     7
 /    / 
1   3 6   9

to

     4
   /   
  7     2
 /    / 
9   6 3   1

题解：

既可以自顶向下，也可以自底向上.

Time Complexity: O(n). Space: O(logn).

AC Java:

/**
 * 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) {
        /*
        //Method 1 自顶向下
        if(root == null){
            return root;
        }
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        invertTree(root.left);
        invertTree(root.right);
        return root;
        */
        
        //Method 2 自底向上
        if(root == null){
            return root;
        }
        TreeNode tempLeft = root.left;
        root.left = invertTree(root.right);
        root.right = invertTree(tempLeft);
        return root;
    }
}

AC JavaScript:

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var invertTree = function(root) {
    if(!root){
        return root;
    }
    
    var l = invertTree(root.left);
    var r = invertTree(root.right);
    root.left = r;
    root.right = l;
    return root;
};