力扣-404题 左叶子之和（C++）- 二叉树+dfs+迭代

题目链接：https://leetcode-cn.com/problems/sum-of-left-leaves/
题目如下：

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;
    }
};