Maximum Size Subarray Sum Equals k

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?

class Solution {

public:

    int maxSubArrayLen(vector<int>& nums, int k) {

        int ans = 0;

        unordered_map<int, int> mp = {{0, -1}};

        

        int sum = 0;

        for (int i = 0; i < nums.size(); i++) {

            int num = nums[i];

            sum += num;

            if (mp.count(sum - k)) {

                ans = max(ans, i - mp[sum - k]);

            }

            if (!mp.count(sum)) {

                mp[sum] = i;

            }

        }

        return ans;

    }

};