LeetCode 437. 路径总和 III Path Sum III (Easy)

/**
 * 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 pathSum(TreeNode* root, int sum) {
        
        if (root == nullptr) return 0;
        int pathNum = pathSumCore(root, sum)  //从当前节点开始
        + pathSum(root->left, sum) + pathSum(root->right, sum);  //从当前节点的子路径开始

        return pathNum;
    }
    int pathSumCore(TreeNode* root, int sum)  //从当前节点开始判断
    {
        if (root == nullptr) return 0;
        int ret = 0;
        if (root->val == sum) ++ret;
        ret += pathSumCore(root->left, sum - root->val) + pathSumCore(root->right, sum - root->val);
        return ret;
    }
};