question:

Modify binary search so that it always returns the element with the smallest index that mathes the search element (and still guarantees logarithmic running time).

answer:

//註意二分查找一定要是有序數組！！！(我搞忘記了，然後怎麽都查不出錯QAQ)

import edu.princeton.cs.algs4.*;

public class BinarySearch
{
    public static
 
 int Rank(int key, int[]a, int lo, int hi)
    {
        if(lo > hi)
            return -1;
        int mid = lo + (hi - lo)/2;
        if(a[mid] == key) 
        {
            int min = Rank(key, a, lo, mid-1);
            if(min != -1)
                return min;
            return mid;
        }
        
 
else if(a[mid] > key)
            return Rank(key, a, lo, mid-1);
        else
            return Rank(key, a, mid+1, hi);
    }

    
    public static void main(String[] args)
    {
        int a[] = {0,0,1,1,2,4,5,5,6,8,9,10};//二分查找必須是有序數組！！！
        int key = 1;
        int ans = Rank(key,a,0,a.length-1);
        StdOut.println(ans);
    }
}
 

1.4.10