437. Path Sum III

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

Problem

You are given a binary tree in which each node contains an integer value.

Find the number of paths that sum to a given value.

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

The tree has no more than 1,000 nodes and the values are in the range -1,000,000 to 1,000,000.

Example:

root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8

      10
     /  \
    5   -3
   / \    \
  3   2   11
 / \   \
3  -2   1

Return 3. The paths that sum to 8 are:

1.  5 -> 3
2.  5 -> 2 -> 1
3. -3 -> 11

Solution

/**
 * 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:
    int pathSum(TreeNode* root, const int sum) {
        if (!root) return 0;
        int count = 0;
        pathSum(count, root, sum, 0);
        return count + pathSum(root->left, sum) + pathSum(root->right, sum);
    }
    void pathSum(int &count, TreeNode* root, const int sum, int cursum) {
        if (!root) return;
        cursum += root->val;
        if (sum == cursum) ++count;
        pathSum(count, root->left, sum, cursum);
        pathSum(count, root->right, sum, cursum);
    }
};

• #dfs

• #important