晨哥Leetcode 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.

解析：
Subarray Sum -> 求出prefix sum
Subarray Sum Equals k -> prefix sum相差为k
Maximum Size -> Map<Integer, Integer> 用hashmap， key是prefix sum，value是index
遇到相同的sum不覆盖（我只要最左边的index，最小的index，这样相差会最大）
prefix sum相差为k的时候就可以计算了
最开始用map.put(0, -1)初始化： 想象一下index为-1， sum初始化为0

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