算法-数组：二分查找

在一个有序(假设升序)无重复元素的数组nums中，寻找元素target，找到了就返回对应的下标，没找到就返回-1。

#include <iostream>
#include <vector>
using namespace std;
//二分查找的前提有两个：有序数组、无重复元素
//“循环不变量规则”：边界要么左闭右闭，要么左闭右开

//方法一：左闭右闭
int method(vector<int>& nums,int target){
    int n = nums.size();
    int left = 0;
    int right = n-1;
    while(left<=right){//因为是左闭右闭，所以此区间内有可能存在left=right的情况
        int middow = (left+right)/2;
        if(target == nums[middow]){
            return middow;
        }
        else if(target < nums[middow]){//在左半边
            right = middow-1;
        }
        else{//在右半边
            left = middow+1;
        }
    }
    return -1;
}

int main(){
    int n;//数组长度
    cin>>n;
    vector<int> nums;
    int num;
    for(int i = 0; i < n; i++){
        cin>>num;
        nums.push_back(num);
    }
    int target;
    cin>>target;
    cout<<"目标值所在下标为："<<method(nums,target);
    
    return 0;
}

#include <iostream>
#include <vector>
using namespace std;

//方法二：左闭右开
int method(vector<int>& nums,int target){
    int n = nums.size();
    int left = 0;
    int right = n-1;
    while(left<right){//因为是左闭右开，所以此区间内不可能存在left=right的情况
        int middow = (left+right)/2;
        if(target == nums[middow]){
            return middow;
        }
        else if(target < nums[middow]){//在左半边
            right = middow;
        }
        else{//在右半边
            left = middow+1;
        }
    }
    return -1;
}

int main(){
    int n;//数组长度
    cin>>n;
    vector<int> nums;
    int num;
    for(int i = 0; i < n; i++){
        cin>>num;
        nums.push_back(num);
    }
    int target;
    cin>>target;
    cout<<"目标值所在下标为："<<method(nums,target);
    
    return 0;
}

标签：二分,num,target,nums,int,算法,查找,include,左闭
来源： https://blog.csdn.net/weixin_60952096/article/details/122658784