LeetCode-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.

Example 1:

Given nums = [1, -1, 5, -2, 3], k = 3,
return 4. (because the subarray [1, -1, 5, -2] sums to 3 and is the longest)

Example 2:

Given nums = [-2, -1, 2, 1], k = 1,
return 2. (because the subarray [-1, 2] sums to 1 and is the longest)

Follow Up:
Can you do it in O(n) time?

 

Analysis:

 

Use a HashMap to store pair of <sum val, ind>, then for each sum[i], we query whether there is <sum[i]-k, x>, if yes, then we found one candidate.

 

Solution:

public class Solution {
    public int maxSubArrayLen(int[] nums, int k) {
        int res = 0, len = nums.length;
        Map<Integer,Integer> sumMap = new HashMap<Integer,Integer>();        

        int sum=0;
        for (int i=0;i<len;i++){
            sum += nums[i];
            if (sum==k) {
                res = i+1;
            } else if (sumMap.containsKey(sum-k)){
                res = Math.max(res, i-sumMap.get(sum-k));
            }
            if (!sumMap.containsKey(sum)){
                sumMap.put(sum,i);
            }    
        }

        return res;
    }
}