在排序数组 X 中搜索第一个索引 i，使得 X[i] >= a

Search in sorted array X for first index i such that X[i] >= a

本文关键字：gt 使得 排序 搜索 第一个 索引 数组      更新时间：2023-10-16

我正在努力适应编码成瘾者在这里提出的代码来解决这个问题的这个简单变体。有人有想法吗？

你采用他的算法，返回high而不是-1。如果X[high]小于您的目标，请改为选择下一项。如果high等于数组大小，则没有这样的索引。

迈克尔·安德森（Michael Anderson）关于你指出的算法是如何编写的是正确的，但它很容易适应。

int find_hi (const std::vector<int> &X, int t) {
   int low  = 0;
   int high = X.size() - 1;
   while(low <= high) {
       int mid = (low + high) / 2;
       if(X[mid] == t) return mid;
       else if(X[mid] < t)
           low = mid + 1;
       else
           high = mid - 1;
   }
   if (high < sz && X[high] < t) ++high;
   return high;
}

但是，这相当于C++中的以下内容：

int find_hi (const std::vector<int> &X, int t) {
    return std::lower_bound(X.begin(), X.end(), t) - X.begin();
}

要查找第一个索引i X[i] >= a，您需要像这样修改结束条件以检查它是否真的是满足条件的第一个索引：

public int findIndex(int[] array, int target) {
    int left = 0;
    int right = array.length;
    while (left < right) {
        int mid = (left + right) / 2;
        if (array[mid] >= target && (mid == 0 || array[mid - 1] < target)) {
            return mid;
        } else if (array[mid] > target) {
            right = mid;
        } else
            left = mid;
    }
    return -1;
}

这是二叉搜索的采用。

#include <list>
#include <iostream>
int arr[9]={0,1,2,3,4,5,7,7,8};
/*int * array - where to search
  int numberOfElements - the number of elements in array
  int targetValue - the value that we are looking for 
*/
int find( int * array, int numberOfElements, int targetValue)
{
    //we start with searching in the whole array (this is our "scope").
    int left = 0;
    int right = numberOfElements;
    while(left < right)
    {
        //We take the middle item of our "scope" and compare it with the target value.
        int mid = (left + right) / 2;
        if( array[ mid ] >= targetValue)//If we did hit
        {
            //we check is it the first item to match our criteria.              
            if(array[ mid - 1 ] < targetValue)
            {
                //If it is, we return the index.
                return mid;
            }
            else
            { 
                //Else, we continue searching to the left. 
                right = mid;
            }
        }
        //If we didnt hit from the first guess, we continue with our "scope"
        //being either the left of the right half of the previous "scope".
        else if( array[mid] > targetValue )
            right = mid;
        else
            left = mid;
     }
}
int main()
{
    std::cout << find(arr, 9, 7);
}

output: 3

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