Kth Smallest Subarray Sum

找第k小的subarray sum.

这题吧..如果不卡时间, 应该不难, 但是这题用treeset过不了, 所以只能用binary search做, 已知subarray sum可以用pre sum求, 并且是递增的 (nums[i] >= 1) 所以这题可以用binary search找一个mid最小值下, 第k的值.

class Solution {
    public int kthSmallestSubarraySum(int[] nums, int k) {
        int l = 0;
        int r = Integer.MAX_VALUE;
        while(l < r)
        {
            int m = (l + r) / 2;
            int f = check(nums, m);
            if(f < k)
                l = m + 1;
            else
                r = m;
        }
        return l;
    }
    public int check(int[] nums, int m){
        int sum = 0;
        int i = 0;
        int j = 0;
        int res = 0;
        while(i < nums.length){
            if(sum + nums[i] <= m){
                sum += nums[i];
                i++;
                res += i - j;
            }
            else
            {
                sum -= nums[j];
                j++;
            }
        }
        return res;
    }
}

class Solution {
    public int kthSmallestSubarraySum(int[] nums, int k) {
        int n = nums.length;
        int[] pre = new int[n + 1];
        for(int i = 1 ; i <= n; i++) {
            pre[i] = pre[i - 1] + nums[i - 1];
        }
        LinkedList<Integer> list = new LinkedList<>();
        for(int i = 0; i <= n; i++){
            for(int j = i + 1; j <= n; j++){
                list.add(pre[j] - pre[i]);
            }
        }
        Collections.sort(list);
        System.out.println(list);
        while(--k != 0){
            list.removeFirst();
        }
        return list.getFirst();
    }
}