我已经实现了快速选择算法。
我遇到的问题是,当我在数组中使用重复项时,我的算法最终会无限循环...
你可以帮助我让它发挥作用吗?预期的复杂度是O(n),最坏情况是O(n ^ 2)?
#include <iostream>
#include <vector>
#include <algorithm>
#include <ctime>
using namespace std;
int rand_partition(vector<int> &a, int left, int right) {
int pivotIndex = left + (rand() % (right - left));
//int m = left + (right - left) / 2; //... to test the algo...no rand at this point
int pivot = a[pivotIndex];
int i = left;
int j = right;
do {
while (a[i] < pivot) i++; // find left element > pivot
while (a[j] > pivot) j--; // find right element < pivot
// if i and j not already overlapped, we can swap
if (i < j) {
swap(a[i], a[j]);
}
} while (i < j);
return i;
}
// Returns the n-th smallest element of list within left..right inclusive (i.e. n is zero-based).
int quick_select(vector<int> &a, int left, int right, int n) {
if (left == right) { // If the list contains only one element
return a[left]; // Return that element
}
int pivotIndex = rand_partition(a, left, right);
// The pivot is in its final sorted position
if (n == pivotIndex) {
return a[n];
}
else if (n < pivotIndex) {
return quick_select(a, left, pivotIndex - 1, n);
}
else {
return quick_select(a, pivotIndex + 1, right, n);
}
}
int main() {
vector<int> vec= {1, 0, 3, 5, 0, 8, 6, 0, 9, 0};
cout << quick_select(vec, 0, vec.size() - 1, 5) << endl;
return 0;
}
答案 0 :(得分:2)
您的代码中存在几个问题。
quick_select()
中,您直接将pivotIndex
与n
进行比较。由于left
并非始终为0,因此您应将n
与左侧部分的长度(pivotIndex - left + 1
进行比较。n > length
时,你只是递归地调用quick_select(a, pivotIndex + 1, right, n)
,此时,它意味着整个向量的第N个元素位于它的右边部分,它是(N) - (pivotIndex - left + 1))向量右边部分的第 - 个元素。代码应该是quick_select(a, pivotIndex + 1, right, n - (pivotIndex - left + 1) )
(n是基于ONE的。)A[p...j] ≤ A[j+1...r]
,但我们希望A[p...j-1] ≤ A[j] ≤ A[j+1...r]
中有quick_select()
。所以我使用基于我在another post上写的Lomuto分区算法的rand_partition()
这是固定的quick_select()
,它返回第n个(注意n是基于ONE )向量的最小元素:
int quick_select(vector<int> &a, int left, int right, int n)
{
if ( left == right )
return a[left];
int pivotIndex = partition(a, left, right);
int length = pivotIndex - left + 1;
if ( length == n)
return a[pivotIndex];
else if ( n < length )
return quick_select(a, left, pivotIndex - 1, n);
else
return quick_select(a, pivotIndex + 1, right, n - length);
}
和this是rand_partition()
:
int rand_partition(vector<int> &arr, int start, int end)
{
int pivot_index = start + rand() % (end - start + 1);
int pivot = arr[pivot_index];
swap(arr[pivot_index], arr[end]); // swap random pivot to end.
pivot_index = end;
int i = start -1;
for(int j = start; j <= end - 1; j++)
{
if(arr[j] <= pivot)
{
i++;
swap(arr[i], arr[j]);
}
}
swap(arr[i + 1], arr[pivot_index]); // swap back the pivot
return i + 1;
}
首先调用srand()
初始化随机数生成器,以便在调用rand()
时可以获得类似随机数的数字。
用于测试上述功能的驱动程序:
int main()
{
int A1[] = {1, 0, 3, 5, 0, 8, 6, 0, 9, 0};
vector<int> a(A1, A1 + 10);
cout << "6st order element " << quick_select(a, 0, 9, 6) << endl;
vector<int> b(A1, A1 + 10); // note that the vector is modified by quick_select()
cout << "7nd order element " << quick_select(b, 0, 9, 7) << endl;
vector<int> c(A1, A1 + 10);
cout << "8rd order element " << quick_select(c, 0, 9, 8) << endl;
vector<int> d(A1, A1 + 10);
cout << "9th order element " << quick_select(d, 0, 9, 9) << endl;
vector<int> e(A1, A1 + 10);
cout << "10th order element " << quick_select(e, 0, 9, 10) << endl;
}