Java二分查找（代码帖）_用java手写一个二分查找

Java二分查找（代码帖）

怕记不住，

对于一个有序数据，可以使用二分查找，前提是数组必须有序，时间复杂度为O(logn)。

二分查找通常有两种写法：递归、非递归

（1）二分查找递归

   /**
     * 二分查找
     */
    public static int binarySerch(int[] array, int value, int left, int right) {
        if(left > right) {
            return -1;
        }
        int middle = (left + right) >>> 1;
        if(array[middle] == value) {
            return  middle;
        }else if(array[middle] > value) {
            return binarySerch(array, value, left, middle-1);
        }else {
            return binarySerch(array, value, middle+1, right);
        }
    }
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

（2）二分查找非递归

/**
 * 二分查找
 * @author lijialin
 *
 */

public class Main {
	
	public static int binarySearch(int[] array, int k) {
		
		int left = 0;
		int right = array.length-1;
		while(left <= right) {
			int mid = (left + right) >> 1;
		    if(array[mid] == k) return mid;
		    else if(array[mid] < k) {
		    	left = mid + 1;
		    }else {
		    	right = mid - 1;
		    }
		}
		return -1;
	}  
	
	public static void main(String[] args) {
		// test case
		int[] array = {1,2,3,4,5,6,7,8,9};
		for(int i = 0; i < array.length; i++) {
			System.out.println(binarySearch(array, array[i]));
		}
	}
	
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34

（3）Collections.binarySearch();

在实际工作中，无需我们自己动手实现二分查找，Collections为我们提供了二分查找方法。

        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(4);
        list.add(5);
        Collections.sort(list);
        int index = Collections.binarySearch(list, 4);
        System.out.println(index);
1
2
3
4
5
6
7
8
9

Collections.binartSearch的源码使用了非递归的方式进行二分查找，对于可以随机访问的存储结构（如数组）、与链式存储结构（如链表）分别进行了实现：

private static <T>
    int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key) {
        int low = 0;
        int high = list.size()-1;

        while (low <= high) {
            int mid = (low + high) >>> 1;
            Comparable<? super T> midVal = list.get(mid);
            int cmp = midVal.compareTo(key);

            if (cmp < 0)
                low = mid + 1;
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }

    private static <T>
    int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)
    {
        int low = 0;
        int high = list.size()-1;
        ListIterator<? extends Comparable<? super T>> i = list.listIterator();

        while (low <= high) {
            int mid = (low + high) >>> 1;
            Comparable<? super T> midVal = get(i, mid);
            int cmp = midVal.compareTo(key);

            if (cmp < 0)
                low = mid + 1;
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41

注：学渣心里苦，不要学楼主，平时不努力，考试二百五，哭 ~