325.Maximum-Size-Subarray-Sum-Equals-k

325. Maximum Size Subarray Sum Equals k

题目地址

https://leetcode.com/problems/maximum-size-subarray-sum-equals-k/

题目描述

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?

代码

Approach #1 HashMap

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