Problem

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.

Examples

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?

Solution

Method 1 - Using Prefix Sum

class Solution {
    public int maxSubArrayLen(int[] nums, int k) {
        Map<Long, Integer> d = new HashMap<>();
        d.put(0L, -1);
        int ans = 0;
        long sum = 0;
        for (int i = 0; i < nums.length; ++i) {
            sum += nums[i];
            ans = Math.max(ans, i - d.getOrDefault(sum - k, i));
            d.putIfAbsent(sum, i);
        }
        return ans;
    }
}