对称二叉树

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
// class Solution {
// public:
//     bool helper(TreeNode* p, TreeNode* q){
//         //终止条件
//         if(!p && !q) return true;
//         if(!p || !q) return false;
//         if(p->val != q->val) return false;
//         //递归逻辑
//         return helper(p->left, q->right) && helper(p->right, q->left);
//     }
//     bool isSymmetric(TreeNode* root) {
//         //若空
//         if(root == nullptr) return true;
//         return helper(root, root);
//     }
// };

//迭代，队列
// class Solution{
// public:
//     bool isSymmetric(TreeNode* root){
//         //若空
//         if(root == nullptr) return true;
//         queue<TreeNode*> que;
//         que.push(root);
//         que.push(root);
//         while(!que.empty()){
//             TreeNode* leftnode = que.front();que.pop();
//             TreeNode* rightnode = que.front();que.pop();

//             if(!leftnode && !rightnode) continue;//都为空
//             if(!leftnode || !rightnode || (leftnode->val != rightnode->val)) return false;
            
//             //压入新值
//             que.push(leftnode->left);
//             que.push(rightnode->right);
//             que.push(leftnode->right);
//             que.push(rightnode->left);
//         }
//         return true;
//     }
// };

//迭代，栈
class Solution{
public:
    bool isSymmetric(TreeNode* root){
        //若空
        if(root == nullptr) return true;
        //
        stack<TreeNode*> stk;
        stk.push(root);
        stk.push(root);
        while(!stk.empty()){
            //左右节点比较
            TreeNode* leftnode = stk.top(); stk.pop();
            TreeNode* rightnode = stk.top();stk.pop();

            //若空
            if(!leftnode && !rightnode) continue;
            //非空
            if(!leftnode || !rightnode || (leftnode->val != rightnode->val)) return false;

            //压入新值
            stk.push(leftnode->left);
            stk.push(rightnode->right);
            stk.push(leftnode->right);
            stk.push(rightnode->left);
        }
        return true;
    }
};

标签：right,return,TreeNode,stk,que,二叉树,对称,leftnode
来源： https://blog.csdn.net/kangjielearning/article/details/114990467