Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.
Input:nums = [1,1,1], k = 2
- The length of the array is in range [1, 20,000].
- The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].
k=sum(i,j)=sum(0,j)-sum(0,i), where sum(i,j) represents the sum of all the elements from index i to j-1.
for each j：how many i < j satisfies sum(0,i) = sum(0,j) - k