325 Maximum Size Subarray Sum Equals k

325. 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.

Note:
The sum of the entire nums array is guaranteed to fit within the 32-bit signed integer range.

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?

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Solution #1
Map里key = [0 - i] 的和，value = i.
因为要找的是最左边的点（与当前i距离最远），所以遇到重复的key可以跳过

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