剑指offer29：最小的k个数

1 题目描述

　　输入n个整数，找出其中最小的K个数。例如输入4,5,1,6,2,7,3,8这8个数字，则最小的4个数字是1,2,3,4,。

2 思路和方法，C++核心代码

2.1 sort()函数，vector<int> push_back(); nlog(n)

class Solution {
public:
    vector<int> GetLeastNumbers_Solution(vector<int> input, int k) {
        sort(input.begin(),input.end());
        vector<int> result;
        if(k>input.size())
            return result;
        for(int i = 0;i<k;i++){
            result.push_back(input[i]);
        }
        return result;
    }
};

2.2 Partition分治O（n）

class Solution {
public:
    vector<int> GetLeastNumbers_Solution(vector<int> input, int k) {
        // 必须检查k的大小否则出错！！！
        if (input.size() == 0 || k > input.size() || k<=0) 
            return vector<int>();
       
        int start = 0;
        int end = input.size() -1;
        int index = Partition(input,start,end);
        while(index != k-1){
            if(index < k-1){
                start = index + 1;  // k在index右边
                index = Partition(input,start,end);
            }
            else{
                end = index -1;     // k在index左边
                index = Partition(input,start,end);
            }
        }
        vector<int> ret;
        for (int i = 0; i < k; ++i) {
            ret.push_back(input[i]);
        }
        return ret;
    }

    int Partition(vector<int> &data, int start, int end){
        if(start > end)
            return start;

        int key = data[start];  // 取第一个值为参考值
        while(start< end){
            while(data[end] >= key && start < end) end--;
            data[start] = data[end];
            while(data[start] <= key && start<end) start++;
            data[end] = data[start];
        }
        data[start] = key;
        return start;
    }
};

参考资料

https://blog.csdn.net/zjwreal/article/details/88608506