public class Solution {
    public int InversePairs(int [] array) {
        if(array.length==0||array==null){
            return 0;
        }
        int[] copyArray=new int[array.length];
        int count=InversePairs(array,copyArray,0,array.length-1);
        return count;
    }
    
    public int InversePairs(int[] array,int[] copyArray,int low,int high){
        if(low==high)
            return 0;
        
        int mid=low+(high-low)/2;
        int i=mid;
        int j=high;
        int localCopy=high;
        int count=0;
        
        int leftCount=InversePairs(array,copyArray,low,mid)%1000000007;
        int rightCount=InversePairs(array,copyArray,mid+1,high)%1000000007;
        
        while(i>=low&&j>mid){
            if(array[i]>array[j]){
                copyArray[localCopy--]=array[i--];
                count+=j-mid;
                if(count>1000000007){
                    count=count%1000000007;
                }
            }else{
                copyArray[localCopy--]=array[j--];
            }
        }
        
        while(i>=low){
            copyArray[localCopy--]=array[i--];
        }
        
        while(j>mid){
            copyArray[localCopy--]=array[j--];
        }
        
        for(int k=low;k<=high;k++){
            array[k]=copyArray[k];
        }
        
        return  (leftCount+rightCount+count)%1000000007;
    }
}

public class Solution {
    public int GetNumberOfK(int [] array , int k) {
        int count=0;
       for(int i=0;i<array.length;i++){
           if(array[i]==k){
               count++;
           }
       }
        return count;
    }
}

二分查找，参考剑指Offer,但是用的非递归。

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;
        }
        
    }
    
    public  int getFirstK(int[] array,int k,int start,int end){
        while(start<=end){
            int mid=(start+end)/2;
            if(k<array[mid])
                end=mid-1;
            else if(k>array[mid])
                start=mid+1;
            else{
                if((mid>0&&array[mid-1]!=k)||mid==0)
                    return mid;
                else{
                    end=mid-1;
                }
            }
        }
        return -1;
    }
    
    public  int getLastK(int[] array,int k ,int start,int end){
        while(start<=end){
            int mid=(start+end)/2;
            if(k<array[mid])
                end=mid-1;
            else if(k>array[mid])
                start=mid+1;
            else{
                if((mid<array.length-1&&array[mid+1]!=k)||mid==array.length-1)
                    return mid;
                else{
                    start=mid+1;
                }
            }
        }
        return -1;
    }
}