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# Shortest Subarray with Sum at Least K

Return the **length** of the shortest, non-empty, contiguous subarray of `A` with sum at least `K`.

If there is no non-empty subarray with sum at least `K`, return `-1`.

1.

**Example 1:**

```
Input: A = [1], K = 1
Output: 1
```

**Example 2:**

```
Input: A = [1,2], K = 4
Output: -1
```

**Example 3:**

```
Input: A = [2,-1,2], K = 3
Output: 3
```

**Note:**

1. `1 <= A.length <= 50000`
2. `-10 ^ 5 <= A[i] <= 10 ^ 5`
3. `1 <= K <= 10 ^ 9`

分析

单调递增栈。 找左边比右边小的，左边i比右边j大的无用，

```
class Solution:
    def shortestSubarray(self, A: List[int], K: int) -> int:
        B = [0]
        for i in A:
            B.append(B[-1]+i)
        s = []
        res = float('inf')
        for i,b in enumerate(B):
            if not s:
                s.append(i)
            else:
                while s and B[s[-1]] > b: s.pop() #useless, pop out
                while s and B[s[0]] <= b - K: 
                    res = min(res,i - s[0])
                    s.pop(0)
                s.append(i)
        return res if res!= float('inf') else -1
            
            
                
            
            
```


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