# leetcode\_325

Given an array nums and a target value k, find the maximum length of a subarray that sums to k. If there isn't one, return 0 instead.

Note: The sum of the entire nums array is guaranteed to fit within the 32-bit signed integer range.

Example 1:

Input: nums = \[1, -1, 5, -2, 3], k = 3 Output: 4 Explanation: The subarray \[1, -1, 5, -2] sums to 3 and is the longest. Example 2:

Input: nums = \[-2, -1, 2, 1], k = 1 Output: 2 Explanation: The subarray \[-1, 2] sums to 1 and is the longest. Follow Up: Can you do it in O(n) time?

## Solutions

1. **prefix sum with hash map O(n)**

```cpp
class Solution {
public:
    int maxSubArrayLen(vector<int>& nums, int k) {
        unordered_map<int, int> m;
        m[0] = -1;
        int cursum = 0, res = 0;
        for (int i = 0; i < nums.size(); i++) {
            cursum += nums[i];
            if (m.count(cursum - k))
                res = max(res, i - m[cursum - k]);
            // record the longest
            if (!m.count(cursum))
                m[cursum] = i;
        }
        return res;
    }
};
```