Invert a binary tree.

     4
   /     2     7
 / \   / 1   3 6   9

     4
   /     7     2
 / \   / 9   6 3   1

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.

對Ｉｔｅｒａｔｉｖｅ做法，類似樹的前序遍歷

/**
 * 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> stack = new Stack<TreeNode>();
        stack.push(root);
        TreeNode tmp, left;
        while (!stack.empty()) {
            tmp = stack.pop();
            if (tmp.left == null && tmp.right == null) {
                continue;
            }
            if (tmp.left == null) {
                tmp.left = tmp.right;
                tmp.right = null;
                stack.push(tmp.left);
                continue;
            }
            if (tmp.right == null) {
                tmp.right = tmp.left;
                tmp.left = null;
                stack.push(tmp.right);
                continue;
            }
            left = tmp.left;
            tmp.left = tmp.right;
            tmp.right = left;
            stack.push(tmp.left);
            stack.push(tmp.right);
        }
        return root;
    }
}

226. Invert Binary Tree