*Leetcode 437. 路径总和 III

Posted 2023-03-05 Z-Pilgrim

https://leetcode.cn/problems/path-sum-iii/

树上前缀和

class Solution 
public:
    void dfs(unordered_map<long long, int> &hash, TreeNode*root, long long cur, long long tar, int &ans) 
        if(!root) return;
        cur += root->val;
        
        if(hash.find(cur - tar ) != hash.end()) 
            ans += hash[cur - tar];
        
        hash[cur] ++;
        dfs(hash, root->left, cur, tar, ans);
        dfs(hash, root->right, cur, tar, ans);
        hash[cur] --;
    

    int pathSum(TreeNode* root, int targetSum) 
        unordered_map<long long, int> hash;
        hash[0]  =1;
        int ans = 0;

        dfs(hash, root, 0, targetSum, ans);
        return ans;
    
;

暴力

class Solution 
public:
    void dfs(TreeNode* root, long long cur, long long target, int &ans) 
        if(!root) 
            return;
        
        if(cur + root->val == target) 
            ans ++;
        ;
        dfs(root->left, cur + root->val, target, ans);
        dfs(root->right, cur + root->val, target, ans);
    

    void dfs(TreeNode* root, long long target, int &ans) 
        if(!root) return;
        dfs(root, 0, target, ans);
        dfs(root->left, target, ans);
        dfs(root->right, target, ans);
    

    int pathSum(TreeNode* root, int targetSum) 
        // dp[i] =p 
        int ans = 0;
        dfs(root, targetSum, ans);
        return ans;
    
;