select.h

来自「常用算法与数据结构原代码」· C头文件 代码 · 共 61 行

// file select.h
// find the k'th smallest element

#ifndef Select_
#define Select_

#include "xcept.h"
#include "swap.h"

template<class T>
T select(T a[], int l, int r, int k)
{// Return k'th smallest in a[l:r].
	if (l >= r) 
		return a[l];
	int i = l,      // left to right cursor
		j = r + 1;  // right to left cursor
	T pivot = a[l];
	
	// swap elements >= pivot on left side
	// with elements <= pivot on right side
	while (true) 
	{
		do {// find >= element on left side
			i = i + 1;
		} 
		while (a[i] < pivot);
		do 
		{// find <= element on right side
			j = j - 1;
		} 
		while (a[j] > pivot);
		if (i >= j) 
			break;  // swap pair not found
		Swap(a[i], a[j]);
	}
	
	if (j - l + 1 == k) 
		return pivot;
	
	// place pivot
	a[l] = a[j];
	a[j] = pivot;
	
	// recursive call on one segment
	if (j - l + 1 < k)
		return select(a, j+1, r, k-j+l-1);
	else return select(a, l, j-1, k);
}


template<class T>
T Select(T a[], int n, int k)
{// Return k'th smallest element in a[0:n-1].
	// Assume a[n] is a dummy largest element.
	if (k < 1 || k > n) 
		throw OutOfBounds();
	return select(a, 0, n-1, k);
}

#endif