LeetCode:Invert Binary Tree

问题描述：

Invert a binary tree.

     4
   /   \
  2     7
 / \   / \
1   3 6   9

to

     4
   /   \
  7     2
 / \   / \
9   6 3   1

思路：

基础题目，提供两个版本：递归和非递归版本

JAVA代码：

递归版本：

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

非递归版本：

/**
 * 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;
        }
        Stack<TreeNode> tnroot = new Stack<TreeNode>(); 
        tnroot.push(root);
        while(!tnroot.isEmpty()){
            TreeNode cur = tnroot.pop();
            TreeNode temp = cur.left;
            cur.left = cur.right;
            cur.right = temp;
            if(cur.left != null){
                tnroot.push(cur.left);
            }
            if(cur.right != null){
                tnroot.push(cur.right);
            }
        }
        return root;
    }
}