二分查找算法

标签：二分查找

提到查找算法，最经典的就是二分查找算法了。在二分查找时要在有序的数据里查找目标target，先取中间元素与target比较，

当target小于中间元素的时候，则搜索数组的前半部分，target大于中间元素时，则取数组的后半部分。重复整个搜索的过程

将左半部分与有半部分当作子数组继续查找，直到找到元素或到子数组的大小为0停止。

原理上很简单却有较多细节,尤其是数据边界的取值是否会越界，while循环的条件。

java code:

public class BinarySearchDemo {
	private int search1(int[] a, int target) {
		int left = 0;
		int right = a.length - 1;
		int mid;
		while (left <= right) {
			mid = left + ((right - left) >> 1);
			if (a[mid] < target) {
				left = mid + 1;
			} else if (a[mid] > target) {
				right = mid - 1;
			} else
				return mid;
		}
		return -1;
	}

	public int search2(int[] a, int target) {
		int left = 0;
		int right = a.length;// 这里取数组大小对后面while循环有影响
		int mid;
		while (left < right) {
			mid = left + ((right - left) >> 1);
			if (target < a[mid])
				right = mid;
			else if (target > a[mid])
				left = mid + 1;
			else
				return mid;
		}
		return -1;
	}

	// 二分法的递归实现
	public int search3(int[] a, int left, int right, int target) {
		if (left > right)
			return -1;
		int mid = left + ((right - left) >> 1);
		if (a[mid] < target) {
			return search3(a, mid + 1, right, target);
		} else if (a[mid] > target) {
			return search3(a, left, right - 1, target);
		} else {
			return mid;
		}
	}

	public static void main(String[] args) {
		int a[] = { 1, 2, 3, 4, 6, 7, 8, 9, 10, 11 };
		int target = 11;
		BinarySearchDemo bs = new BinarySearchDemo();
		int pos1 = bs.search1(a, target);
		System.out.println("search1:" + pos1);
		int pos2 = bs.search2(a, target);
		System.out.println("search2:" + pos2);
		int pos3 = bs.search3(a, 0, a.length, target);
		System.out.println("search3:" + pos3);
	}
}

二分查找算法

标签：二分查找

原文地址：http://blog.csdn.net/lgcssx/article/details/38827641