22、查找算法-二分法查找

 1 //二分法查找
 2 public class BinarySearch {
 3     public static void main(String[] args) {
 4         int[] arr = {1,2,3,4,5,6,7,8,9,10};
 5 
 6         int a = binarySearch(arr,0,arr.length,8);
 7         System.out.println(a);
 8     }
 9 
10     public static int binarySearch(int[] arr,int left,int right,int finalVal){
11         if(left > right){
12             return -1;
13         }
14         int mid = (left + right) / 2;
15         int midVal = arr[mid];
16         if(finalVal > midVal){
17             return binarySearch(arr,mid+1,right,finalVal);
18         }else if(finalVal < midVal){
19             return binarySearch(arr,left,mid-1,finalVal);
20         }else {
21             return mid;
22         }
23     }
24 
25 }

7

 1     public static ArrayList<Integer> binarySearch2(int[] arr, int left, int right, int finalVal){
 2         if(left > right){
 3             return new ArrayList<Integer>();
 4         }
 5         int mid = (left + right) / 2;
 6         int midVal = arr[mid];
 7         if(finalVal > midVal){
 8             return binarySearch2(arr,mid+1,right,finalVal);
 9         }else if(finalVal < midVal){
10             return binarySearch2(arr,left,mid-1,finalVal);
11         }else {
12             ArrayList<Integer> resIndexList = new ArrayList<Integer>();
13             //向左
14             int temp = mid - 1;
15             while (true){
16                 if(temp < 0 || arr[temp] != finalVal){
17                     //左边到头了，或者左边的没有相等的
18                     break;
19                 }else {
20                     resIndexList.add(temp);
21                     temp -= 1;
22                 }
23             }
24 
25             //当前
26             resIndexList.add(mid);
27 
28             //向右
29             temp = mid + 1;
30             while (true){
31                 if(temp > arr.length - 1 || finalVal != arr[temp]){
32                     //右边到头了，或者右边的没有相等的
33                     break;
34                 }else {
35                     resIndexList.add(temp);
36                     temp += 1;
37                 }
38             }
39             return resIndexList;
40         }
41     }

1         int[] arr = {1,2,3,4,5,6,7,8,9,10,10,10,10};
2 
3         int a = binarySearch(arr,0,arr.length,8);
4         System.out.println(a);
5 
6         ArrayList<Integer> b = binarySearch2(arr,0,arr.length,10);
7         System.out.println(b);

7
[9, 10, 11, 12]