使用递归算法进行二分搜索

Question

Help me. I am a newbie to java and I am stuck in binary search recursion (stackoverflow error) and I can't find solution anywhere.

public class BinarySearch {
    public static int binAry (int ary[], int st, int lt, int val){
        if (st<=lt) {
            int mid = (st + (lt-st)) / 2;
            if (val > ary[mid]) {
                binAry(ary, mid+1, lt, val);
            }
            else if (val < ary[mid]) {
                binAry(ary, st, mid-1, val);
            }
            else{
                return mid;
            }
        }
        return -1;
    }
    public static void main (String[] args){
        BinarySearch bs = new BinarySearch();
        int [] numbers = {0,1,2,3,4,5};
        int x = 3;
        int pt = numbers.length;
        int p = bs.binAry(numbers, 0, pt-1, x);
        if (p == -1){
            System.out.println("Number not found.");
        }
        else{
            System.out.println("Number found at " + p);
        }
    }
}

Answer 1

For what I see, the problem is that (val > ary[mid]) will always evaluate to true. Always. Thus your code will enter an endless loop, which, ultimately, results in a StackOverFlow exception.

If you follow the value of the variables involved in your code to that point, you will notice that val never changes, being its value 3 since the beginning.

Also, int mid = (st + (lt-st)) / 2 is the same as lt/2 because st + (lt - st) = st - st + lt (do the test by assigning them whatever values you want). So even though you change the value of st in binAry(ary, mid+1, lt, val) , it really has no effect. lt will always be numbers.length - 1 and mid will always be 5/2=2.

Add this code right below int mid = (st + (lt-st)) / 2 to check for yourself:

System.out.println("st="+st);    
System.out.println("lt="+lt);
System.out.println("mid="+mid);

Maybe if you explained what you want to do I would understand this a bit better and perhaps I'd be able to actually help find a solution.

Edit: Ok, after dedicating it a bit more time I found your error. Change the formula:

int mid = (st + (lt-st)) / 2  // This is wrong
int mid = st + (lt-st) / 2;  // This is ok, notice the parentheses