算法（第4版）-1.1.10 二分查找

import java.util.Arrays;

public class BinarySearch {
    public static int rank(int key, int[] a) {
        int lo = 0;
        int hi = a.length - 1;
        while (lo <= hi) {
            // 被查找的键要么不存在，要么必然存在于a[lo..hi]之中
            int mid = lo + (hi - lo) / 2;
            if (key < a[mid])
                hi = mid - 1;
            else if (key > a[mid])
                lo = mid + 1;
            else
                return mid;
        }
        return -1;
    }

    public static void main(String[] args) {
        int[] a = {1, 3, 5, 7, 9, 11};
        int key = 5;
        System.out.println(rank(key, a));
        key = 8;
        System.out.println(rank(key, a));
    }
}