leetcode题记:Symmetric Tree-蒲公英云

leetcode题记:Symmetric Tree

编程语言：JAVA

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 [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
Note:
Bonus points if you could solve it both recursively and iteratively.

提交反馈：

193 / 193 test cases passed.
Status: Accepted
Runtime: 16 ms
Submitted: 1 minute ago
You are here! 
Your runtime beats 1.44 % of java submissions

解题思路：

题目判断树是否对称，借助上一道题【Same Tree】的思路，将根节点分成左右两个数，分别判断左右树是否对称。
判断结束条件为：当前节点都为空，返回true；
当前节点只有一个为空，返回false；
当前值不相等，返回false；
isSame(p.left,q.right) && isSame(p.right,q.left);
这类型题目最主要的还是掌握递归思想吧，掌握了递归思想，简单题目大多都是递归思想。

代码：

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null){
            return true;
        }
        TreeNode p = root.left;
        TreeNode q = root.right;
        return isSame(p,q);
    }
    public boolean isSame(TreeNode p, TreeNode q){
        if(p == null && q == null){
            return true;
        }
        if(p == null || q == null){
            return false;
        }
        System.out.println(p.val + ":" + q.val);
        if(p.val != q.val){
            return false;
        }
        return isSame(p.left,q.right) && isSame(p.right,q.left);
    }
}