// Time:  O(n)
// Space: O(n)

class Solution {
public:
    int maxSubArrayLen(vector<int>& nums, int k) {
        unordered_map<int, int> sums;
        int cur_sum = 0, max_len = 0;
        for (int i = 0; i < nums.size(); ++i) {
            cur_sum += nums[i];
            if (cur_sum == k) {
                max_len = i + 1;
            } else if (sums.find(cur_sum - k) != sums.end()) {
                max_len = max(max_len, i - sums[cur_sum - k]);
            }
            if (sums.find(cur_sum) == sums.end()) {
                sums[cur_sum] = i;  // Only keep the smallest index.
            }
        }
        return max_len;
    }
};