二叉树的镜像(简单) yinshibanxian

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

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

输入：root = [4,2,7,1,3,6,9]
输出：[4,7,2,9,6,3,1]

0 <= 节点个数 <= 1000

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var mirrorTree = function(root) {
    preOrderTraverse(root);
    return root;
};

var preOrderTraverse = function(node) {
    if(node != null) {
        let temp = node.left;
        node.left = node.right;
        node.right = temp;
        preOrderTraverse(node.left);
        preOrderTraverse(node.right);
    }
}