二分查找

程序包括查找第一个和最后一个出现的数字位置。

#include <iostream>#include <vector>using namespace std;int BinarySearch(vector<int> &num,int left,int right,int key){    if(left > right)        return -1;    int mid = (left+right)/2;    if(num[mid] == key)        return mid;         //如果要找第一个或者最后一个出现的key，在这个if里再细分    else if(num[mid] < key)        return BinarySearch(num,mid+1,right,key);    else        return BinarySearch(num,left,mid-1,key);    return -1;}//查找第一个出现的keyint BinarySearchFirst(vector<int> &num,int left,int right,int key){    if(left > right)        return -1;    int mid = (left+right)/2;    if(num[mid] == key){        if(mid==left || num[mid-1]!=key)            return mid;        else            return BinarySearch(num,left,mid-1,key);    }    else if(num[mid] < key)        return BinarySearch(num,mid+1,right,key);    else        return BinarySearch(num,left,mid-1,key);    return -1;}//查找最后一个出现的keyint BinarySearchLast(vector<int> &num,int left,int right,int key){    if(left > right)        return -1;    int mid = (left+right)/2;    if(num[mid] == key){        if(mid==right || num[mid+1]!=key)            return mid;        else            return BinarySearch(num,mid+1,right,key);    }    else if(num[mid] < key)        return BinarySearch(num,mid+1,right,key);    else        return BinarySearch(num,left,mid-1,key);    return -1;}int main(){    int a[] = {1,1,2,3,4,5,5,5,7,8,12,15,16,23};    vector<int> input(begin(a),end(a));    int left = 0;    int right = input.size()-1;    int find = BinarySearch(input,left,right,5);    int find1 = BinarySearchFirst(input,left,right,5);    int find2 = BinarySearchLast(input,left,right,5);    cout<<find<<endl;    cout<<find1<<endl;    cout<<find2<<endl;    system("pause");    return 0;}