面试题 04.12. Paths with Sum LCCI

Problem

You are given a binary tree in which each node contains an integer value (which might be positive or negative). Design an algorithm to count 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).

Example

Given the following tree and sum = 22,

Output:

3
Explanation: Paths that have sum 22 are: [5,4,11,2], [5,8,4,5], [4,11,7]

Solution

注意：路径不必从树根开始，到叶节点结束。

2020-3-4 dfs递归，日后再看优化方法。

/**
 * 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:
    void  dfs(TreeNode* root,int sum,int &curSum,int &pathNum)
    {
        if(!root)
            return;
        
        curSum += root->val;
        if(curSum == sum)
        {
            ++pathNum;
        }

        if(root->left)
            dfs(root->left,sum,curSum,pathNum);
        if(root->right)
            dfs(root->right,sum,curSum,pathNum);
            
        curSum -= root->val;

    }

    int pathSum(TreeNode* root, int sum) {
        if(!root)
            return 0;

        int curSum = 0;
        int pathNum = 0;

        dfs(root,sum,curSum,pathNum);

        pathNum += pathSum(root->left,sum);
        pathNum += pathSum(root->right,sum);

        return pathNum;

    }


};