【9月打卡~Leetcode每日一题】437. 路径总和 III（难度：中等）

437. 路径总和 III

树的前缀和 时间复杂度O(N)

/**
 * 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:
    unordered_map<int, int> hashmap;

    int dfs(TreeNode* root,int cur, int targetSum) {
        int ret = 0;
        if(! root){
            return ret;
        }
        cur += root -> val;
        if(hashmap[cur - targetSum]){
            ret += hashmap[cur - targetSum];
        }
        hashmap[cur] ++;
        ret += dfs(root -> left, cur, targetSum);
        ret += dfs(root -> right, cur, targetSum);
        hashmap[cur] --; //重点！！

        return ret;
    }

    int pathSum(TreeNode* root, int targetSum) {
        hashmap[0] = 1;
    
        return dfs(root,0,targetSum);
    }
};