[LintCode] 61. Search for a Range_Easy tag: Binary Search

Posted 2021-01-02 johnsonxiong

 

Description

Given a sorted array of n integers, find the starting and ending position of a given target value.

If the target is not found in the array, return [-1, -1].

Example

Given [5, 7, 7, 8, 8, 10] and target value 8,
return [3, 4].

Challenge

O(log n) time.

这个题目的思路就是用两个binary search, 分别求first index 和last index.

Code   其实可以利用helper funciton来去优化code.

class Solution:
    def searchRange(self, A, target):
        # write your code here
        if not A: return [-1,-1]
        l, r , ans = 0, len(A) -1, [-1,-1]
        while l + 1 < r:
            mid = l + (r - l)//2
            if A[mid] < target:
                l = mid
            elif A[mid] > target:
                r = mid
            else:
                r = mid
        if A[l] == target:
            ans[0] = l
        elif A[r] == target:
            ans[0] = r
        else:
            return ans

        # find last index
        l, r = 0, len(A) -1
        while l + 1 < r:
            mid = l + (r - l)//2
            if A[mid] < target:
                l = mid
            elif A[mid] > target:
                r = mid
            else:
                l = mid
        if A[r] == target:
            ans[1] = r
        elif A[l] == target:
            ans[1] = l
        else:
            return ans
        return ans