【算法】求一个数字在给定的已排序数组中出现的起始、终止索引号_用函数求某个值所在的起始 终止序号

方法一

public class findthefirstandlast {
    public int finderfirstandlast(int[] arr, int target) {
        int first = findfirstindex(arr, target);
        int last = findlastindex(arr, target);
        System.out.println(first);
        System.out.println(last);
        return findlastindex(arr, target) - findfirstindex(arr, target);
    }

    public int findfirstindex(int[] arr, int target) {
        int left = 0;
        int right = arr.length - 1;
        int mid;
        while (left <= right) {
            mid = (left + right) / 2;
            if (arr[mid] < target) {
                left = mid + 1;
            } else if (arr[mid] > target) {
                right = mid - 1;

            } else {
                if ( mid == 0 || arr[mid - 1] != target  ) {
                    return mid;
                }
                right = mid - 1;
            }
        }
        return -1;
    }

    public int findlastindex(int[] arr, int target) {
        int left = 0;
        int right = arr.length - 1;
        int mid;
        while (left <= right) {
            mid = (left + right) / 2;
            if (arr[mid] < target) {
                left = mid + 1;
            } else if (arr[mid] > target) {
                right = mid - 1;

            } else {
                if (mid == arr.length - 1 || arr[mid + 1] != target) {
                    return mid;
                }
                left = mid + 1;
            }
        }
        return -1;
    }
}


1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53

方法二：
https://www.cnblogs.com/kyoner/p/11080078.html

参考链接
https://blog.csdn.net/lonelymanontheway/article/details/80330560
http://www.mamicode.com/info-detail-184617.html