C++ 代码

/**
 * 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:
int sum=0;
    int sumOfLeftLeaves(TreeNode* root) {
        if(!root){
            return 0;
        }
        if (isLeaf(root->left))
        {
            sum+=root->left->val;
        }

     sumOfLeftLeaves(root->left) ;
      sumOfLeftLeaves(root->right);
      return sum;
    }
    bool isLeaf(TreeNode* node){
if (!node) return false;
return !node->left  && !node->right;
} 
};