剑指offer——37.数字在排序数组中出现的次数

题目描述

统计一个数字在排序数组中出现的次数。

思路：
二分查找第一次出现的位置和最后一次出现的位置，相减即可。

代码：

public class Solution {
    public int GetNumberOfK(int [] array , int k) {
        if(array == null || array.length == 0)
            return 0;
        int first = getFirstK(array,k,0,array.length - 1);
        int last = getLastK(array,k,0,array.length - 1);
        if(first == -1 || last == -1)
            return 0;
        else
            return last - first + 1;
    }
    private int getFirstK(int [] array, int k, int low, int high){
        int mid = 0;
        while(low <= high){
            mid = low + (high -low) / 2;
            if(array[mid] == k){
                if(mid > 0 && array[mid - 1] != k || mid == 0)
                    return mid;
                else
                    high = mid - 1;
            }
            else if(array[mid] > k)
                high = mid - 1;
            else
                low = mid + 1;
        }
        return -1;
    }
    private int getLastK(int [] array, int k, int low, int high){
        int mid = 0;
        while(low <= high){
            mid = low + (high -low) / 2;
            if(array[mid] == k){
                if(mid < array.length - 1 && array[mid + 1] != k || mid == array.length - 1)
                    return mid;
                else
                    low = mid + 1;
            }
            else if(array[mid] > k)
                high = mid - 1;
            else
                low = mid + 1;
        }
        return -1;
    }
}