Java [Leetcode 101]Symmetric Tree

题目描述：

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

解题思路：

递归法

代码如下：

/**
 * Definition for a binary tree node.
 * 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;
        return  isMirror(root.left, root.right);
    }

    public boolean isMirror(TreeNode left, TreeNode right){
    	if(left == null && right == null)
    		return true;
    	else if(left == null)
    		return false;
    	else if(right == null)
    		return false;
    	else{
    		if(left.val != right.val)
    			return false;
    		return isMirror(left.right, right.left) && isMirror(left.left, right.right);
    	}
    }
}