101. Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
镜面条件,根节点左右子树：
1、left.val == right.val
2、left的左子树和right的右子树成镜面
3、left的右子树和right的左子树成镜面

public class Solution {   public boolean isSymmetric(TreeNode root) {        if (root == null) {            return true;        } else {            return isSame(root.left, root.right);        }    }    private boolean isSame(TreeNode left, TreeNode right) {        if(left == null && right == null) return true;        if(left == null || right == null) return false;        return (left.val == right.val) && isSame(left.left,right.right) && isSame(left.right,right.left);    }}