【leetcode】523. Continuous Subarray Sum

Given a list of non-negative numbers and a target integer k, write a function to check if the array has a continuous subarray of size at least 2 that sums up to the multiple of k, that is, sums up to n*k where n is also an integer.

Example 1:

Input: [23, 2, 4, 6, 7],  k=6
Output: True
Explanation: Because [2, 4] is a continuous subarray of size 2 and sums up to 6.

 

Example 2:

Input: [23, 2, 6, 4, 7],  k=6
Output: True
Explanation: Because [23, 2, 6, 4, 7] is an continuous subarray of size 5 and sums up to 42.

 

Note:

1. The length of the array won't exceed 10,000.
2. You may assume the sum of all the numbers is in the range of a signed 32-bit integer.

解题思路：用map存储当前序列的和跟终点下标

class Solution{
    public boolean checkSubarraySum(int[] nums, int k) {
        if(nums == null) {
            return false;
        }
        Map<Integer,Integer> map = new HashMap<>();
        map.put(0,-1);
        int sum = 0;
        for(int i = 0;i < nums.length;i++) {
            sum += nums[i];
            if(k != 0)
                sum %= k;
            Integer prev = map.get(sum);
            if(prev != null) {
                if(i - prev > 1)
                    return true;
            }else 
                map.put(sum,i);
        }
        return false;
    }
}