二分查找算法

二分查找算法

Java版二分查找算法

二分查找算法的目标查找集合应该为有序序列

/*
* @(#)BinarySearch.java 2009-8-8
*
* Copyright (c) 2009 by jadmin. All Rights Reserved.
*/

package algorithm.search;

/**
* 二分查找算法
*
* @author <a href="mailto:[email protected]">jadmin</a>
* @version $Id: BinarySearch.java 2009-8-8 上午05:07:05$
* @see <a href="http://hi.baidu.com/jadmin">myblog</a>
*/
public final class BinarySearch {

public static int find(int[] a, int key) {
   return find(a, 0, a.length - 1, key);
}

// 非递归实现
public static int find(int[] a, int fromIndex, int toIndex, int key) {
   int low = fromIndex;
   int high = toIndex;

   while (low <= high) {
    // 无符号右移位逻辑运算
    int mid = (low + high) >>> 1;
    int midVal = a[mid];

    if (midVal < key)
     low = mid + 1;
    else if (midVal > key)
     high = mid - 1;
    else
     return mid; // key found
   }
   return -(low + 1); // key not found.
}

// 递归实现
public static int search(int[] a, int fromIndex, int toIndex, int key) {
   if(fromIndex > toIndex) {
    return -1;
   }
   int mid = (fromIndex + toIndex) >>> 1;
   if(a[mid] < key) {
    return search(a, mid + 1, toIndex, key);
   } else if(a[mid] > key) {
    return search(a, fromIndex, mid - 1, key);
   } else {
    return mid;
   }
}