Java面试--常见的边界控制问题

常见的边界控制问题

二分查找

例 【二分查找】  在有序数组中查找元素 k ，返回 k 所在的下标

在数组 [1,2,10,15,100] 中去寻找 15 ，15 的下标是 3 ；

二分查找思路：

代码实现：BinarySearch.java

package interview.loop;

public class BinarySearch {

  /**
   * Searches element k in a sorted array.
   * @param arr a sorted array
   * @param k the element to search
   * @return index in arr where k is. -1 if not found.
   */
  public int binarySearch(int[] arr, int k) {	//传入参数数组
    int a = 0;			//定义数组下标 a、b
    int b = arr.length;		//数组下标包含整个数组
	//二分查找要保证的条件
    // Loop invariant: [a, b) is a valid range. (a <= b)
    // k may only be within range [a, b).
    while (a < b) {
      int m = a + (b - a) / 2; // m = (a + b) / 2 may overflow! 防止内存溢出
      if (k < arr[m]) {		//k 与中间节点 m 做比较，要查找的数据在 m 的左边
        b = m;
      } else if (k > arr[m]) {	//要查找的数据在 m 的右边
        a = m + 1;
      } else {
        return m;
      }
    }
    return -1;		//查找不到则返回 -1
  }

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

    System.out.println("Testing normal data");	
    System.out.println(
        bs.binarySearch(new int[]{1, 2, 10, 15, 100}, 15));
    System.out.println(
        bs.binarySearch(new int[]{1, 2, 10, 15, 100}, -2));
    System.out.println(
        bs.binarySearch(new int[]{1, 2, 10, 15, 100}, 101));
    System.out.println(
        bs.binarySearch(new int[]{1, 2, 10, 15, 100}, 13));
    System.out.println("======");

    System.out.println("Testing empty or singleton data.");
    System.out.println(
        bs.binarySearch(new int[]{}, 13));
    System.out.println(
        bs.binarySearch(new int[]{12}, 13));
    System.out.println(
        bs.binarySearch(new int[]{13}, 13));
    System.out.println("======");

    System.out.println("Testing data of size 2.");
    System.out.println(
        bs.binarySearch(new int[]{12, 13}, 13));
    System.out.println(
        bs.binarySearch(new int[]{12, 13}, 12));
    System.out.println(
        bs.binarySearch(new int[]{12, 13}, 11));
  }
}

输出：