LeetCode-226.Invert Binary Tree

Invert a binary tree.

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

to

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

Trivia:
This problem was inspired by  this original tweet  by  Max Howell :

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

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

 */

递归：

public TreeNode InvertTree(TreeNode root)
    {
        if(root==null)
            return null;
        TreeNode l=root.left;
        TreeNode r=root.right;
        root.left=InvertTree(r);
        root.right=InvertTree(l);
        return root;
    }

非递归

借助队列 DFS

public TreeNode InvertTree(TreeNode root)
    {
        if (root == null)
                return null;
            Queue<TreeNode> q = new Queue<TreeNode>();
            q.Enqueue(root);
            TreeNode cur,tmp;
            while (q.Count>0)
            {
                cur = q.Dequeue();
                tmp = cur.left;
                cur.left = cur.right;
                cur.right = tmp;
                if (cur.left != null)
                    q.Enqueue(cur.left);
                if (cur.right != null)
                    q.Enqueue(cur.right);
            }
            return root;
    }

或者栈 BFS

public TreeNode InvertTree(TreeNode root)
    {
        if (root == null)
                return null;
            Stack<TreeNode> q = new Stack<TreeNode>();
            q.Push(root);
            TreeNode cur,tmp;
            while (q.Count>0)
            {
                cur = q.Pop();
                tmp = cur.left;
                cur.left = cur.right;
                cur.right = tmp;
                if (cur.left != null)
                    q.Push(cur.left);
                if (cur.right != null)
                    q.Push(cur.right);
            }
            return root;
    }