LeetCode--search-for-a-range

最新推荐文章于 2019-08-24 22:48:00 发布

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最新推荐文章于 2019-08-24 22:48:00 发布

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分类专栏： LeetCode 文章标签： LeetCode C++

本文链接：https://blog.youkuaiyun.com/wl_ss/article/details/79046309

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本文介绍了一种使用二分查找算法来确定已排序整数数组中特定目标值起始和结束位置的方法。该算法确保了时间复杂度为O(log n)，并提供了一个C++实现示例。

题目描述

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

Your algorithm's runtime complexity must be in the order ofO(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].

分析：使用二分查找，查找左右边界即可

class Solution {
public:
vector<int> searchRange(int A[], int n, int target) {
  vector<int> res(2,-1);
  if(A==nullptr || n<=0)
    return res;
  int low = 0,high = n-1;
  while(low<=high)
  {
      int middle = (high+low)>>1;
      if(A[middle] < target)   /// 向左夹逼，则找到左边界
         low = middle+1;
      else
         high = middle-1;
  }
 
  int low2 = 0,high2 = n-1;
  while(low2 <=high2)
  {
      int middle2 = (high2+low2)>>1;
      if(A[middle2] <= target) /// 向右夹逼，找到右边界
         low2 = middle2+1;
      else
        high2 = middle2-1;
  }
  if(low <= high2)
  {
      res[0] = low;
      res[1] = high2;
  }
  return res;
    }
};