数字在排序数组中出现的次数

题目

　　统计一个数字在排序数组中出现的次数。

分析

　　利用二分查找，找到该数字第一次出现的位置和最后一次出现的位置。

代码

　　

    public int GetNumberOfK(int[] array, int k){
        int first = GetFirst(array, k, 0, array.length-1);
        int last = GetLast(array, k, 0, array.length-1);
        if(first==-1 || last==-1)
            return 0;
        return last-first+1;
    }
    
    public int GetFirst(int[] array, int k, int low, int high){
        int tag = -1;
        
        while(low<=high){
            int mid = (low+high)/2;
            if(array[mid]>k){
                high = mid-1;
            }
            else if(array[mid]<k){
                low = mid+1;
            }
            else{
                if(mid==0 || (mid>0 && (array[mid-1]<array[mid]))){
                    tag = mid;
                    break;
                }
                else{
                    high = mid-1;
                }
            }
        }
        return tag;
    }
    
    public int GetLast(int[] array, int k, int low, int high){
        int tag = -1;
        while(low<=high){
            int mid = (low+high)/2;
            if(array[mid]>k)
                high = mid-1;
            else if(array[mid]<k)
                low = mid+1;
            else{
                if(mid==array.length-1 || (mid<array.length-1 && (array[mid]<array[mid+1]))){
                    tag = mid;
                    break;
                }
                else{
                    low = mid+1;
                }
            }
        }
        return tag;
    }