二分法基本查找

public class Kedaxunfei02 {
    public static void main(String[] args) {
        int [] a = {11,13,15,17,19,21};
        System.out.println(binarySearch01(a, 21));  
    }
    public static int binarySearch01(int[] a,int key) {
        int start = 0;
        int end = a.length-1;
        while (start<=end){
            int mid = (start+end)/2;
            if(key<a[mid]) 
                end = mid-1;
            else if(key>a[mid])
                start = mid+1;
            else 
                return mid;
        }
        //没找到返回-1
        return -1;
    }
}

二分法遍历查找

public class Kedaxunfei02 {
    public static void main(String[] args) {
        int [] a = {11,13,15,17,19,21};
        System.out.println(binarySearch02(a, 21,0,a.length-1)); 
    }
    
    public static int binarySearch02(int[] a,int key,int start,int end) {
        while (start<=end){
            int mid = (start+end)/2;
            if(key<a[mid]) 
                return erfenfa(a, key, start, mid-1);
            else if(key>a[mid]) 
                return erfenfa(a, key, mid+1, end);
            else 
                return mid;
        }
        return -1;
    }
}