题目链接/文章讲解/视频讲解：https://programmercarl.com/0404.%E5%B7%A6%E5%8F%B6%E5%AD%90%E4%B9%8B%E5%92%8C.html

1.还是先递归，明显感觉跟上这个算法的思路了。

CPP

/**
 * 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 traveral(TreeNode * node){
        if(node == NULL) return 0;
        if(node->left == NULL && node->right == NULL) return 0;

        int leftSum = traveral(node->left);
        

        if(node->left != NULL && node->left->left == NULL && node->left->right ==NULL){
            leftSum = node->left->val;
        }
        int rightSum = traveral(node->right);
        int sum = leftSum + rightSum;
        return sum;
    }

    int sumOfLeftLeaves(TreeNode* root) {
        return traveral(root);
    }
};