算法基础-二分查找

CYX_cheng2024-09-19 19:45

左闭右闭 $left，right$ $1,1$ 可以

while( left <= right )
if( a $mid$ > target )
right = mid - 1
else if( a $mid$ < target )
left = mid + 1

左闭右开 [ left，right ) [1，1)不可以

while( left < right )
if( a $mid$ > target ) 已经>target，开区间right = mid
right = mid
else if( a $mid$ < target )
left = mid + 1

java 复制代码

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int m = in.nextInt();
        int l, r;
        int[] a = new int[n];
        for(int i = 0; i < n; i ++)
            a[i] = in.nextInt();

        while(m -- > 0) {
            int first = -1, last = -1;
            int x = in.nextInt();
            l = 0;
            r = n - 1;
            while(l <= r) {
                int mid = (l + r) / 2;
                if(a[mid] > x) {
                    r = mid - 1;
                } else if(a[mid] < x) {
                    l = mid + 1;
                } else {
                    first = mid;
                    r = mid - 1;
                }
            }
            l = 0;
            r = n - 1;
            while(l <= r) {
                int mid = (l + r) / 2;
                if(a[mid] > x) {
                    r = mid - 1;
                } else if(a[mid] < x) {
                    l = mid + 1;
                } else {
                    last = mid;
                    l = mid + 1;
                }
            }
            System.out.println(first + " " + last);
        }
    }
}