Floor.java

public class Floor {
    public static void main(String[] args) {
        int[] arr = {2, 3, 5, 9, 14, 16, 18};
        int ans = floor(arr,15);
        System.out.println(ans);
    }

    // return the index of greates no smaller than equal to target
    static int floor(int[] arr, int target){
        int start=0;
        int end=arr.length-1;

        while(start<=end){
            // find the middle element
//            int mid = (start +end)/2;  // might be possible that (start+end) exceeds the range of int in java
            int mid = start + (end-start)/2;

            if (target<arr[mid]){
                end=mid-1;
            } else if (target>arr[mid]) {
                start=mid+1;
            }
            else{
                // ans found
                return mid;
            }
        }
        return end;
    }
}