LeetCode "437. Path Sum III"

The punch line of this one: sum of leaves: pi..pj = root..pj - root..pi.

/**
 * 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 {
    int ret;
    void go(TreeNode *p, int sofar, int tgt, unordered_map<int, unsigned> rec)
    {
        if(!p) return;
        
        sofar += p->val;
        if(sofar == tgt)
        {
            ret ++; // case 1 : from root
        }
        
        int t = sofar - tgt;
        if(rec.find(t) != rec.end())
        {
            ret += rec[t];
        }
        
        rec[sofar] ++;
        go(p->left, sofar, tgt, rec);
        go(p->right,sofar, tgt, rec);
    }
public:
    int pathSum(TreeNode* root, int sum) 
    {
        ret = 0;
        if(!root) return 0;
        
        go(root, 0, sum, unordered_map<int, unsigned>());
        return ret;
    }
};