Leetcode325-maximumSizeSubarraySumEqualsK

Description

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

Example 1:

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3

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:

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3

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

class Solution {
    public int maxSubArrayLen(int[] nums, int k) {
        HashMap<Integer, Integer> map = new HashMap<>();
        int sum = 0;
        int res = 0;
        for (int i = 0; i < nums.length; i++){
            sum += nums[i];
            if (sum == k) res = i + 1;
            else if (map.containsKey(sum - k))
                res = Math.max(res, i - map.get(sum - k));
            if (!map.containsKey(sum)) map.put(sum, i);    
        }
        return res;
    }
}