101. Symmetric Tree（js）

101. Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / 
  2   2
 /  / 
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / 
  2   2
      
   3    3
题意：给定的二叉搜索树是否对称
代码如下：

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isSymmetric = function(root) {
        return !root || isLeftToRight(root.left,root.right);
    
    }
var isLeftToRight = function(left, right){
        if(!left || !right) return left==right;
        if(left.val!=right.val) return false;
        
        return isLeftToRight(left.left,right.right) && isLeftToRight(left.right,right.left);
    }