leetcode – 34. Search for a Range – Medium

descrition

Given an array of integers sorted in ascending order, find the starting and ending position of a given target value.

Your algorithm’s runtime complexity must be in the order of O(log n).

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

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

解析

对于有序数组的查找问题，基本上都可以使用折半查找的思路。再者题目的要求复杂度是 O(log n)，因此我们需要两次折半查找。如果我们只用一次折半查找，找到数组 target 出现的任意一个位置，然后线性遍历找到最左边和最右，需要的复杂度是 O(n)。

具体实现代码如下。

code

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

class Solution{
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        vector<int> ans(2, -1);
        int ileft = binarySearchLeftMost(nums, target);
        if(ileft < 0)
            return ans;
        
        int iright = binarySearchRightMost(nums, target);

        ans[0] = ileft;
        ans[1] = iright;

        return ans;
    }

    int binarySearchLeftMost(vector<int>& nums, int target){
        int ans = -1; // save the left most index in nums which equal to target
        int ileft = 0, iright = nums.size() - 1;

        while(ileft <= iright){
            int imid = ileft + (iright - ileft) / 2;
            if(nums[imid] == target){
                ans = imid;
                iright = imid - 1;
            }else if (nums[imid] < target){
                ileft = imid + 1;
            }else{
                // nums[imid] > target
                iright = imid - 1;
            }
        }

        return ans;
    }

    int binarySearchRightMost(vector<int>& nums, int target){
        int ans = -1;
        int ileft = 0, iright = nums.size() - 1;

        while(ileft <= iright){
            int imid = ileft + (iright - ileft) / 2;
            if(nums[imid] == target){
                ans = imid;
                ileft = imid + 1;
            }else if (nums[imid] < target){
                ileft = imid + 1;
            }else {
                // nums[imid] > target
                iright = imid - 1;
            }
        }

        return ans;
    }


};

int main()
{
    return 0;
}

作者：fanling999 链接：https://www.cnblogs.com/fanling999/