[leetcode 560. Subarray Sum Equals K]

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.

Example 1:

Input:nums = [1,1,1], k = 2
Output: 2

Note:

1. The length of the array is in range [1, 20,000].
2. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

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笨方法就是遍历所有子序列，由于起点和终点都要遍历，所以时间复杂度是O（n^2）

本次的方法是遍历所有起点是最开始元素的子序列，我们起名叫sum[i]，只要两个sum[i]的差值是k，则两个i作为起点和终点的子序列就符合要求。由于起点就是最开始元素，只需遍历终点，时间复杂度为O（n）。

public class Solution {
    public int subarraySum(int[] nums, int k) {
        Map<Integer,Integer> map = new HashMap<Integer,Integer>();
        map.put(0,1);
        int sum = 0;
        int res = 0;
        for(int i = 0;i<nums.length;i++)
        {
            sum += nums[i];
            res += map.getOrDefault(sum-k,0);
            map.put(sum,map.getOrDefault(sum,0)+1);
            
        }
        return res;
    }
}