LeetCode Search for a Range

rosepicker

于 2017-01-13 16:33:11 发布

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分类专栏： leetcode 文章标签： leetcode

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本文介绍了一种算法，可以在已排序的整数数组中找到特定目标值的起始和结束位置，利用二分搜索提高查找效率，并确保算法的时间复杂度为O(log n)。

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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].

思路：进行二分搜索，当找到目标值时，在已该元素为中心进行扩展，从而确定区域范围

代码如下：

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        int middle,front,back;
        int start=-1,end=-1;
        vector<int> result;
        front = 0,back=nums.size()-1;
        middle = (front+back)/2;
        while(front<=back)
        {
            if(nums[middle] > target)
            {
                back=middle-1;
                middle=(front+back)/2;
            }
            else if(nums[middle] < target)
            {
                front = middle+1;
                middle=(front+back)/2;
            }
            else
            {
                start=end=middle;
                while(start>0 && nums[start-1] == target)
                    start--;
                while(end < nums.size()-1 && nums[end+1] == target)
                    end++;
                break;
            }
        }
        result.push_back(start);
        result.push_back(end);
        return result;
    }
};