66.Subarray Sum Equals K (K is the number of sub-arrays, and a)

Level：

  Medium

Subject description:

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].

Analysis of ideas:

  Given an array of integers and a number k, which need to find the sum of the number k of successive subarray solved sum [i, j] = the number of target, obtained sum [0, i] and the sum [0, j] can know the sum [i, j]. Because we require all sum (0, i) = sum (0, j) - k of the sum (0, i), then if sum (0, i1) = sum (0, i2), you can directly save a value sum (0, i) is equal to the number of sub-array count value, and then use a HashMap saved.

Code:

public class Solution{
    public int subarraySum(int []nums,int k){
        if(nums==null||nums.length==0)
            return 0;
        int res=0;
        HashMap<Integer,Integer>map=new HashMap<>();//键保存sum(0,i)，值表示其相同值出现的次数
        map.put(0,1);
        int sum=0;
        for(int i=0;i<nums.length;i++){
            sum=sum+nums[i];  //表示sum(0,j)
            if(map.containsKey(sum-k)){
                res=res+map.get(sum-k);
            }
            map.put(sum,map.getOrDefault(sum,0)+1);
        }
        return res;
    }
}