1.2.9 BinarySearch

package search;

public class BinarySearch{

    public static int rank(int key, int[] a) {
        return rank(key, a, 0, a.length - 1);
    }
    
    /**
     * 根据首尾元素的比较结果判断数组的升降序
     * @param key
     * @param a
     * @param lo
     * @param hi
     * @return
     */
    public static int rank(int key, int[] a, int lo, int hi) {
        return a[lo] < a[hi] ? rankAsc(key, a, lo, hi) : rankDesc(key, a, lo, hi);
    }
    
    /**
     * 对升序数组进行查找
     * @param key
     * @param a
     * @param lo
     * @param hi
     * @return
     */
    public static int rankAsc(int key, int[] a, int lo, int hi) {
        while(lo <= hi){
            int mid = (lo + hi) / 2;
            if(key < a[mid]) hi = mid - 1;
            else if(key > a[mid]) lo = mid + 1;
            else return mid;
        }       
        return -1;
    }
    
    /**
     * 对降序数组进行查找
     * @param key
     * @param a
     * @param lo
     * @param hi
     * @return
     */
    public static int rankDesc(int key, int[] a, int lo, int hi) {
        while(lo <= hi){
            int mid = (lo + hi) / 2;
            if(key > a[mid]) hi = mid - 1;
            else if(key < a[mid]) lo = mid + 1;
            else return mid;
        }       
        return -1;
    }

    /**
     * 返回匹配键的最小索引
     * @param key
     * @param a
     * @return
     */
    public static int rankFirstIndex(int key, int[] a) {
        int lo = 0;
        int hi = a.length - 1;
        int index = -1;
        
        while(lo <= hi){
            int mid = (lo + hi) / 2;
            if(key < a[mid]) hi = mid - 1;
            else if(key > a[mid]) lo = mid + 1;
            else{index = mid; break;}
        }
        
        while(index > 0){
            if(a[index-1] != a[index]) break;
            else index--;
        }
        
        return index;
    }
}