good good study, day day up!

vendor/gopl.io/ch4/graph/main.go

@@ -0,0 +1,43 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 99.
+
+// Graph shows how to use a map of maps to represent a directed graph.
+package main
+
+import "fmt"
+
+//!+
+var graph = make(map[string]map[string]bool)
+
+func addEdge(from, to string) {
+	edges := graph[from]
+	if edges == nil {
+		edges = make(map[string]bool)
+		graph[from] = edges
+	}
+	edges[to] = true
+}
+
+func hasEdge(from, to string) bool {
+	return graph[from][to]
+}
+
+//!-
+
+func main() {
+	addEdge("a", "b")
+	addEdge("c", "d")
+	addEdge("a", "d")
+	addEdge("d", "a")
+	fmt.Println(hasEdge("a", "b"))
+	fmt.Println(hasEdge("c", "d"))
+	fmt.Println(hasEdge("a", "d"))
+	fmt.Println(hasEdge("d", "a"))
+	fmt.Println(hasEdge("x", "b"))
+	fmt.Println(hasEdge("c", "d"))
+	fmt.Println(hasEdge("x", "d"))
+	fmt.Println(hasEdge("d", "x"))
+
+}