LeetCode 404. Sum of Left Leaves（统计左叶子节点的和）

题意：统计左叶子节点的和。

/**
 * 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:
    bool leftleaf(TreeNode *root){
        if(root == NULL) return false;
        return root -> left == NULL && root -> right == NULL;
    }
    int sumOfLeftLeaves(TreeNode* root) {
        if(root == NULL) return 0;
        if(leftleaf(root -> left)) return root -> left -> val + sumOfLeftLeaves(root -> right);
        else return sumOfLeftLeaves(root -> left) + sumOfLeftLeaves(root -> right);
    }
};