LeetCode 101 Symmetric Tree

题目

题解

非递归

/**
 * 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) {
        if(!root) return true;
        stack<TreeNode*> sk;
        sk.push(root->left);
        sk.push(root->right);
        TreeNode* pA, *pB;
        while(!sk.empty()) {
            pA = sk.top();
            sk.pop();
            pB = sk.top();
            sk.pop();
            if(!pA && !pB) continue;
            if(!pA || !pB) return false;
            if(pA->val != pB->val) return false;
            sk.push(pA->right);
            sk.push(pB->left);
            sk.push(pA->left);
            sk.push(pB->right);
        }

        return true;
    }
};

递归

/**
 * 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 isSymmetricRecursively(struct TreeNode* Lnode ,struct TreeNode* Rnode ){
        if(Lnode==NULL && Rnode==NULL)
            return true;
        if(Lnode==NULL || Rnode==NULL)
            return false ;
        return ((Lnode->val==Rnode->val) &&
                    isSymmetricRecursively(Lnode->left,Rnode->right) 
                               && isSymmetricRecursively(Lnode->right,Rnode->left )) ;
    }

    bool isSymmetric(struct TreeNode* root) {
        if(root==NULL)
            return true;
        return isSymmetricRecursively(root->left ,root->right); 
    }
};