325. Maximum Size Subarray Sum Equals k

LeetCode 325. Maximum Size Subarray Sum Equals k

Description

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

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.

Constraints:

• 1 <= nums.length <= 2 * 105

• -104 <= nums[i] <= 104

• -109 <= k <= 109

Tags

Hash Table

Solution

This is a prefix-sum problem, combining LeetCode 560. Subarray Sum Equals K and 525. Contiguous Array. Note that we only record the map from prefix-sum to its index when such prefix-sum does not exist in the map, since we need the longest subarray.

Complexity

• Time complexity: $O(n)$

• Space complexity: $O(n)$

Code

func maxSubArrayLen(nums []int, k int) int {
	ans, preSum, sum2idx := 0, 0, map[int]int{0: -1}
	for i, num := range nums {
		preSum += num
		if preIdx, ok := sum2idx[preSum-k]; ok {
			if ans < i-preIdx {
				ans = i - preIdx
			}
		}
		if _, ok := sum2idx[preSum]; !ok {
			sum2idx[preSum] = i
		}
	}
	return ans
}