962. Maximum Width Ramp

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Given an array A of integers, a ramp is a tuple (i, j) for which i < j and A[i] <= A[j].  The width of such a ramp is j - i.

Find the maximum width of a ramp in A.  If one doesn't exist, return 0.

Example 1:

Input: [6,0,8,2,1,5]
Output: 4
Explanation: 
The maximum width ramp is achieved at (i, j) = (1, 5): A[1] = 0 and A[5] = 5.

Example 2:

Input: [9,8,1,0,1,9,4,0,4,1]
Output: 7
Explanation: 
The maximum width ramp is achieved at (i, j) = (2, 9): A[2] = 1 and A[9] = 1.

Note:

1. 2 <= A.length <= 50000
2. 0 <= A[i] <= 50000

思路：维护一个单调递减数组，然后每来一个数，二分法找比这个数小的数的位置

import bisect
class Solution(object):
    def maxWidthRamp(self, A):
        """
        :type A: List[int]
        :rtype: int
        """
        res = 0
        q = [-A[0]]
        q_index = [0]
        for i in range(1,len(A)):
            idx = bisect.bisect_left(q, -A[i])
            if idx!=len(q):
                res = max(res, i-q_index[idx])
            else:
                q.append(-A[i])
                q_index.append(i)
        return res