[JAVA]寻找满足和的最短子序列（Minimum Size Subarray Sum）

题目来源：leetcode

题目描述：

Given an array of n positive integers and a positive integer s, find the minimal length of a contiguous subarray of which the sum ≥ s. If there isn't one, return 0 instead.

Example: 

Input: s = 7, nums = [2,3,1,2,4,3]
Output: 2
Explanation: the subarray [4,3] has the minimal length under the problem constraint.

Follow up:

If you have figured out the O(n) solution, try coding another solution of which the time complexity is O(n log n). 

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我的解法：第一个想到的就是递归和分治，每次头坐标向后移一位找到当前的最短子序列，然后与之前的对比，取不为0的最小值。但此方法繁琐，不推荐使用

class Solution {
    public int minSubArrayLen(int s, int[] nums) {
        return sub(0,s,nums);//开始寻找
    }
    public int sub(int i,int s,int[] nums)//i为当前其实坐标，s和值，nums数组
    {
        if(i>=nums.length) return 0;//当达到数组尾端返回0
        int count=0,sum=0;
        for(int j=i;j<nums.length;++j)//遍历当前数组找到大于等于和值的位数
        {
            if(sum<s) {sum+=nums[j]; ++count;}
            else break;
        }
        if(sum<s) count=0;//如果找不到返回0
        int countnext=sub(i+1,s,nums);//移位后的数组函数
        if(countnext==0) return count;
        if(count<countnext) return count;
        else return countnext;
        
    }
}

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其他解法：遍历数组的同时，前端和尾端是同时移动的，当达到和值时，先保存位数，前端向后移一位，移到不满足条件时，后端开始移动。同样每次移动之后保存的都是满足条件的最小值

class Solution {
    public int minSubArrayLen(int s, int[] nums) {
        if (nums == null || nums.length == 0) {
            return 0;
        }
        int N = nums.length;
        int min = Integer.MAX_VALUE;
        int left = 0;
        int sum = 0;
        
        for(int i = 0; i < N; i++) {
            sum += nums[i];
            while(sum >= s) {
                min = Math.min(min, i - left + 1);
                if(min == 1) {
                    return min;
                }
                sum -= nums[left++];//前端向后移动
            }
        }
        
        return min != Integer.MAX_VALUE ? min : 0;
    }
}