Problem

Given an array of n integer, and a moving window(size k), move the window at each iteration from the start of the array, 
find the sum of the element inside the window at each moving.

Example
For array [1,2,7,8,5], moving window size k = 3. 
1 + 2 + 7 = 10
2 + 7 + 8 = 17
7 + 8 + 5 = 20

return [10,17,20]

思路

• questions
  • nums could be null?
  • k could be <= 0? or >=nums.length
• 

    public int[] winSum(int[] nums, int k) {
        if (nums == null || nums.length == 0 || k <= 0) {
            return new int[0];
        }
        int n = nums.length;
        int[] ans = new int[n - k + 1];
        int[] ps = new int[k + 1];

        for (int i = 1; i <= n; i++) {
            ps[i] = ps[i - 1] + nums[i - 1];
            if (i >= k) {
                ans[i - k] = ps[i] - ps[i - k];
            }
        }
        return ans;
    }