java查找算法之二分查找

查找算法之二分查找

二分查找，一个既简单，又实用，又容易被所有面试官问到的问题，可以可以

以下分为查找一个值和查找多个相同 的值

package cn.ycl.dataStructures.Search;
public class BinarySearch {
  public static void main(String[] args) {
    // TODO Auto-generated method stub
    int arr[] = { 1, 4, 7, 9, 12, 90, 98 };
    int binarySearch = binarySearch(arr, 0, arr.length-1, 12);
    System.out.println(binarySearch);
  }
  public static int binarySearch(int arr[], int left, int right, int findVal) {
    if (left>right) {
      return -1;
    }
    int mid = (left + right) / 2;
    
    
    if (arr[mid] < findVal) {
      left = mid + 1;
      return binarySearch(arr, left, right, findVal);
    } else if (arr[mid]> findVal) {
      right = mid - 1;
      return binarySearch(arr, left, right, findVal);
    } else {
      return mid;
    }
  }
}


//以下为同时查找所有等于要查找的值，
package cn.ycl.dataStructures.Search;
import java.util.ArrayList;
import java.util.List;
public class BinarySearch {
  public static void main(String[] args) {
    // TODO Auto-generated method stub
    int arr[] = { 1, 4, 7, 9, 12, 12, 90, 98 };
    List<Integer> binarySearch = binarySearch(arr, 0, arr.length - 1, 12);
    System.out.println(binarySearch.size());
    for (Integer integer : binarySearch) {
      System.out.println(integer);
    }
  }
  public static List<Integer> binarySearch(int arr[], int left, int right, int findVal) {
    if (left > right) {
      return null;
    }
    int mid = (left + right) / 2;
    if (arr[mid] < findVal) {
      left = mid + 1;
      return binarySearch(arr, left, right, findVal);
    } else if (arr[mid] > findVal) {
      right = mid - 1;
      return binarySearch(arr, left, right, findVal);
    } else {
      
      ArrayList<Integer> list = new ArrayList<Integer>();
      //相mid的左边扫描
      int temp=mid-1;
      while (true) {
        if (temp<0|| arr[temp] !=findVal) {
          break;
        }
        list.add(temp);
        temp -=1;
      }
      list.add(mid);
      //向mid的右边扫描
      temp=mid+1;
    
      while (true) {
        if (temp>arr.length-1 || arr[temp] !=findVal) {
          break;
        }
        list.add(temp);
        temp +=1;
      }
    
      return list;
    }
  }
}