每天刷個演算法題20160525:快速排序的遞迴轉非遞迴解法
阿新 • • 發佈:2019-01-27
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為了防止思維僵化,每天刷個演算法題。已經刷了幾天了,現在發點程式碼。
我已經建了一個開源專案,每天的題目都在裡面:
絕大部分演算法都是我自己寫的,沒有參考網上通用程式碼。讀者可能會覺得有的程式碼晦澀難懂,因為那是我自己的理解。
最近幾天都是在寫一些原來的東西,大多數是非遞迴。以後準備刷點DP、貪心之類的題。
下面是快速排序的遞迴轉非遞迴解法。
是不是沒有人像我這樣蛋疼地把快速排序寫成非遞迴的???
/** * * Copyright 2016 Xiaofei * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. * */ package xiaofei.algorithm; import java.util.Stack; /** * Created by Xiaofei on 16/5/25. * */ public class QuickSort { private static int[] array; private static void sortCorecursively(int l, int r) { class Element { int l; int r; int i; int state; //這裡用state表示通用狀態,和之前非遞迴程式碼風格不一樣。 Element(int l, int r) { this.l = l; this.r = r; this.i = -1; this.state = 0; } } Stack<Element> stack = new Stack<>(); stack.push(new Element(l, r)); while (!stack.isEmpty()) { Element element = stack.peek(); if (element.state == 0) { int i = element.l, j = element.r, m = array[(i + j) / 2]; do { while (array[i] < m) { ++i; } while (m < array[j]) { --j; } if (i <= j) { int temp = array[i]; array[i] = array[j]; array[j] = temp; ++i; --j; } } while (i <= j); element.i = i; if (element.l < j) { stack.push(new Element(element.l, j)); } element.state = 1; } else if (element.state == 1) { if (element.i < element.r) { stack.push(new Element(element.i, element.r)); } element.state = 2; } else if (element.state == 2) { stack.pop(); } } } public static void sort(int[] array) { QuickSort.array = array; sortCorecursively(0, array.length - 1); } }