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 [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.

题目思路

代码 C++

• 思路一、递归实现

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        // 树为空也是对称的
        if(root == NULL){
            return true;
        }
        return core(root->left, root->right);
    }
    
    bool core(TreeNode* zuo, TreeNode* you){
        if(zuo == NULL && you == NULL){
            return true;
        }
        
        if(zuo == NULL || you == NULL){
            return false;
        }
        
        if(zuo->val == you->val){
            return core(zuo->left, you->right) && core(zuo->right, you->left);
        }
        else{
            return false;
        }
    }
};

总结展望