Go语言圣经练习5.11

前言

练习5.11： 现在线性代数的老师把微积分设为了前置课程。完善topSort，使其能检测有向图中的环。

没找到5.11的参考Go语言版，参考C语言版本的思路，结合原有的代码逻辑，在最小改动的原则上实现了Go语言版本的深度优先遍历拓扑排序环检测。
5.10、5.12 可参考 https://blog.csdn.net/taoshihan/article/details/101249100
参考的C语言版本 https://blog.csdn.net/qq_40507857/article/details/84305444

实现逻辑

用三种状态来标记访问的节点：未访问、正在访问、已访问；
初始情况，均为未访问；
未访问项访问时设置为正在访问，递归访问该项的前序课程，访问结束后设置为已访问；
若正在访问时遇到一个正在访问的课程，说明存在环，标记，退出。

代码

package main

import (
	"fmt"
	"sort"
)

func topoSort(m map[string][]string) map[int]string {
	var order = make(map[int]string)
 	index := 0
	seen := make(map[string]int) // 初始值默认为0， 与下面notvisit状态相对应
	const (
	    notvisit = iota
		visiting  //正在访问
		visited   // 访问结束
	)
	hasRing := false
	var visitAll func(items []string)

	visitAll = func(items []string) {
		if hasRing {
			return 
		}
		for _, item := range items {
			if seen[item] == notvisit{
				seen[item] = visiting 
				//fmt.Printf("visiting: %v\n", item)
				visitAll(m[item])
				order[index] = item 
				index++
				seen[item] = visited
			}else if seen[item] == visiting {
				//fmt.Printf("already visit: %v\n", item)
				hasRing = true
				return 
			}
		}
	}

	var keys []string
	for key := range m {
		keys = append(keys, key)
	}

	visitAll(keys) 
	if hasRing {
		fmt.Printf("found ring\n");
		order = make(map[int]string)
	}
	return order
}

func main() {
	results = topoSort(prereqs)
	for i:=0; i<len(results); i++ {
		fmt.Printf("%d:\t%s\n", i+1, results[i])
	}
}

var prereqs = map[string][]string{
	"algorithms": {"data structures"},
	"calculus":   {"linear algebra"},
	"linear algebra": {"calculus"},
	"compilers": {
		"data structures",
		"formal languages",
		"computer organization",
	},

	"data structures":       {"discrete math"},
	"databases":             {"data structures"},
	"discrete math":         {"intro to programming"},
	"formal languages":      {"discrete math"},
	"networks":              {"operating systems"},
	"operating systems":     {"data structures", "computer organization"},
	"programming languages": {"data structures", "computer organization"},
}

输出

case 1 prereqs 加入 “linear algebra”: {“calculus”},

 found ring

case 2 prereqs 不加入 “linear algebra”: {“calculus”},

1:      intro to programming
2:      discrete math
3:      data structures
4:      algorithms
5:      calculus
6:      linear algebra
7:      formal languages
8:      computer organization
9:      operating systems
10:     networks
11:     programming languages
12:     compilers
13:     databases