力扣-404题左叶子之和（C++）- 二叉树+dfs+迭代

傷城~ 2022-09-17 03:03 149阅读 0赞

题目链接：[https://leetcode-cn.com/problems/sum-of-left-leaves/][https_leetcode-cn.com_problems_sum-of-left-leaves]  
题目如下：  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBASkluZ2xlczEyMw_size_20_color_FFFFFF_t_70_g_se_x_16]  
递归：

/**
     * 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:
        int sumOfLeftLeaves(TreeNode* root) {
        
            int sum=0;
            dfs(root,sum);
            return sum;
        }
    
        void dfs(TreeNode* root,int& sum){
        
            if(root==NULL) return ;
            
            //存在左叶子节点：如果当前左节点不为空，且左节点没有左右孩子节点
            if(root->left&&root->left->left==NULL&&root->left->right==NULL){
        
                sum+=root->left->val;
            }
    
            if(root->left) dfs(root->left,sum);
            if(root->right) dfs(root->right,sum);
        }
    };

迭代：

/**
     * 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:
        int sumOfLeftLeaves(TreeNode* root) {
        
            int sum=0;
            stack<TreeNode*> stk;
            if(root==NULL) return sum;
    
            stk.push(root);
            while(!stk.empty()){
        
                TreeNode* temp=stk.top();
                stk.pop();
    
                if(temp->left&&temp->left->left==NULL&&temp->left->right==NULL)
                sum+=temp->left->val;
    
                if(temp->left) stk.push(temp->left);
                if(temp->right) stk.push(temp->right);
            }
    
            return sum;
        }
    };

[https_leetcode-cn.com_problems_sum-of-left-leaves]: https://leetcode-cn.com/problems/sum-of-left-leaves/
[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBASkluZ2xlczEyMw_size_20_color_FFFFFF_t_70_g_se_x_16]: /images/20220828/6d53def4a9dc40e1a75133e3289f9480.png