剑指offer 38 数字在排序数组中出现的次数

自己的写法

class Solution {
public:
    int GetNumberOfK(vector<int> data ,int k) {
        int length = data.size();
        if(length <= 0)
            return 0;
        for(int i = 0;i < length;i++){
            
        }
        int index1 = GetFirst(data,k,0,length-1);
        int index2 = GetLast(data,k,0,length-1);
        return exit(data,k) ? (index2-index1+1) : 0;       
    }
    int GetFirst(vector<int> data,int k,int start,int end){
        if(start == end)
            return start;
        int first = 0;
        int mid = (start+end)/2;
        if(data[mid] == k){
            if(data[mid-1] != k)
                return mid;
            else
                first = GetFirst(data,k,start,mid-1);    
        }
        else if(data[mid] < k)
            first = GetFirst(data,k,mid+1,end);
        else
            first = GetFirst(data,k,start,mid-1);
        return first;
    }
    int GetLast(vector<int> data,int k,int start,int end){
        if(start == end)
            return start;
        int last = end;
        int mid = (start+end)/2;
        if(data[mid] == k){
            if(data[mid+1] != k)
                return mid;
            else
                last = GetLast(data,k,mid+1,end);    
        }
        else if(data[mid] < k)
            last = GetLast(data,k,mid+1,end);
        else
            last = GetLast(data,k,start,mid-1);
        return last;
    }
    bool exit(vector<int> data,int k){
        int length = data.size();
        int i = 0;
        for(;i < length;i++){
            if(data[i] == k)
                return true;
        }
        return false;
    }
};

更简洁的代码

class Solution {
public:
    int GetNumberOfK(vector<int> data ,int k) {
        if(data.empty() || k <= 0)
            return 0;
        int FirstK = GetFirstK(data,k,0,data.size()-1);
        int LastK = GetLastK(data,k,0,data.size()-1);
        if(FirstK > -1 && LastK > -1)
            return LastK - FirstK + 1;
        else
            return 0;
    }
    int GetFirstK(vector<int> data,int k,int start,int end){
        if(start > end)
            return -1;
        int mid = (end + start)/2;
        if(data[mid] == k){
            if(data[mid-1] == k)
                end = mid - 1;
            else
                return mid;
        }
        else if(data[mid] > k)
            end = mid - 1;
        else if(data[mid] < k)
            start = mid + 1;
        return GetFirstK(data,k,start,end);
    }
    int GetLastK(vector<int> data,int k,int start,int end){
        if(start > end)
            return -1;
        int mid = (end + start)/2;
        if(data[mid] == k){
            if(data[mid+1] == k)
                start = mid + 1;
            else
                return mid;
        }
        else if(data[mid] > k)
            end = mid - 1;
        else if(data[mid] < k)
            start = mid + 1;
        return GetLastK(data,k,start,end);
        
    }
};

 用循环的方法做：

注意一个问题，end只能等于mid - 1，不能等于mid，同样begin只能等于mid + 1，不能等于mid。如果换成mid，当只有两个数的时候，会陷入死循环！

class Solution {
public:
    int GetNumberOfK(vector<int> data ,int k) {
        int length = data.size();
        if(length <= 0)
            return 0;
        int begin = FindFirstK(data,k);
        int end = FindLastK(data,k);
        if(begin != -1 && end != -1 )
            return end - begin + 1;
        else
            return 0;
    }
    int FindFirstK(vector<int> data,int k){
        int length = data.size();
        int begin = 0;
        int end = length-1;
        while(begin < end){
            int mid = (begin + end)/2;
            if(data[mid] == k && data[mid-1] != k)
                return mid;
            else if(data[mid] == k && data[mid-1] == k)
                end = mid - 1;
            else if(data[mid] > k)
                end = mid - 1;
            else if(data[mid] < k)
                begin = mid + 1;
        }
        if(data[begin] == k)
            return begin;
        else
            return -1;
    }
    int FindLastK(vector<int> data,int k){
        int length = data.size();
        int begin = 0;
        int end = length-1;
        while(begin < end){
            int mid = (begin + end)/2;
            if(data[mid] == k && data[mid+1] != k)
                return mid;
            else if(data[mid] == k && data[mid+1] == k)
                begin = mid + 1;
            else if(data[mid] > k)
                end = mid - 1;
            else
                begin = mid + 1;
        }
        if(data[begin] == k)
            return begin;
        else
            return -1;
    }
};