查找

二分查找

二分查找的特殊情况1

若arr[mid] == 1, min = mid + 1;
若arr[mid] != 1, max = mid - 1;

二分查找的特殊情况2

三分查找

二分查找代码实现：

#include <stdio.h>

#define P(func, args...){\
    printf("%s = %d\n", #func, func(args));\
}

//1 3 5 7 9 10 
int binary_search1(int *num, int n, int x){
    int head = 0, tail = n - 1, mid;
    while(head <= tail){
        mid = (head + tail) >> 1;
        if(num[mid] == x) return mid;
        if(num[mid] < x) head = mid + 1;
        else tail = mid - 1;
    }
    return -1;
}  

//11111111110000000000
int binary_search2(int *num, int n){
    int head = -1, tail = n - 1, mid;
    while(head < tail){
        mid = (head + tail + 1) >> 1;
        if(num[mid] == 1) head = mid;
        else tail = mid - 1;
    }
    return head;
}

//00000000011111111111
int binary_search3(int *num, int n){
    int head = 0, tail = n, mid;
    while(head < tail){
        mid = (head + tail) >> 1;
        if(num[mid] == 1) tail = mid;
        else head = mid + 1;
    }
    return head == n ? -1 : head;
}


int main(){
    int arr1[10] = {1, 3, 5, 7, 9, 11, 13, 17, 19, 21};
    int arr2[10] = {1, 1, 1, 1, 0, 0, 0, 0, 0, 0};
    int arr3[10] = {0, 0, 0, 0, 0, 0, 1, 1, 1, 1};
    P(binary_search1, arr1, 10, 7);
    P(binary_search2, arr2, 10);
    P(binary_search3, arr3, 10);
    return 0;
}

来源：CSDN

作者：algo▪Tempest

链接：https://blog.csdn.net/weixin_43899266/article/details/104416780