Symmetric Tree

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

For example, this binary tree is symmetric:

    1   / \  2   2 / \ / \3  4 4  3

But the following is not:

    1   / \  2   2   \   \   3    3

递归解法：

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isSymmetric(TreeNode root) {        if(root==null)        {            return true;        }        else         {            return isMirror(root.left,root.right);        }    }    public boolean isMirror(TreeNode left,TreeNode right)    {        if(left==null||right==null)//注：不能写成left==null&&right==null，报错        {            return left==right;        }        else        {            return (left.val==right.val)&&isMirror(left.left,right.right)&&isMirror(left.right,right.left);        }    }}

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