二分查找

二分查找：

package arraystolist;

/**
 * 二分查找
 * 
 * @author hanxiaofan
 *
 */
public class BinarySearch {

	/**
	 * 
	 * @param arr
	 *            待查询数组
	 * @param k
	 *            查询数字
	 * @return
	 */
	public int binarySearch(int[] arr, int k) {
		int a = 0;
		int b = arr.length;
		while (a < b) {
			int m = a + (b - a) / 2; // [a,b)
			if (k < arr[m]) { // 如果k小于中间数
				b = m;
			} else if (k > arr[m]) {
				a = m + 1;
			} else {
				return m;
			}
		}
		return -1;
	}

	public static void main(String[] args) {
		BinarySearch bs = new BinarySearch();

		System.out.println(bs.binarySearch(new int[] { 1, 2, 3, 4, 5 }, 4));
		System.out.println(bs.binarySearch(new int[] {}, 4));

		System.out.println(bs.binarySearch(new int[] { 1, 3, 5 }, 4));
	}
}