437. Path Sum III

Given the root of a binary tree and an integer targetSum, return the number of paths where the sum of the values along the path equals targetSum.

The path does not need to start or end at the root or a leaf, but it must go downwards (i.e., traveling only from parent nodes to child nodes).

Example 1:

Example 1 Tree

Input: root = [10,5,-3,3,2,null,11,3,-2,null,1], targetSum = 8
Output: 3
Explanation: The paths that sum to 8 are shown.

Example 2:

Input: (root = [5, 4, 8, 11, null, 13, 4, 7, 2, null, null, 5, 1]),
  (targetSum = 22);
Output: 3;

Solution

Details

class Solution {
public:
    unordered_map<long, int> mp; // sum, number of paths
    int ans = 0;
    int pathSum(TreeNode* root, int targetSum) {
        ans = 0;
        mp[0] = 1;
        dfs(root, targetSum, 0);
        return ans;
    }
    void dfs(TreeNode* root, int target, long sum) {
        if (!root)
            return;
        sum += root->val;
        if (mp.find(sum - target) != mp.end())
            ans += mp[sum - target];
        mp[sum]++;
        dfs(root->left, target, sum);
        dfs(root->right, target, sum);
        mp[sum]--;
    }
};

Path Sum Sum of Left Leaves