go: add «1408. String Matching in an Array»

URL:	https://leetcode.com/problems/string-matching-in-an-array/
Signed-off-by: Matej Focko <me@mfocko.xyz>

go/string-matching-in-an-array.go

@@ -0,0 +1,54 @@
+package main
+
+type TrieNode struct {
+	frequency int
+	children  map[rune]*TrieNode
+}
+
+func NewNode() *TrieNode {
+	return &TrieNode{
+		frequency: 0,
+		children:  make(map[rune]*TrieNode),
+	}
+}
+
+func (node *TrieNode) Insert(word string) {
+	n := node
+	for _, c := range word {
+		_, ok := n.children[c]
+		if !ok {
+			n.children[c] = NewNode()
+		}
+
+		n = n.children[c]
+		n.frequency++
+	}
+}
+
+func (node *TrieNode) IsSubstring(word string) bool {
+	n := node
+	for _, c := range word {
+		n = n.children[c]
+	}
+
+	return n.frequency > 1
+}
+
+func stringMatching(words []string) []string {
+	matching := make([]string, 0)
+
+	root := NewNode()
+	for _, word := range words {
+		for start := 0; start < len(word); start++ {
+			root.Insert(word[start:])
+		}
+	}
+
+	for _, word := range words {
+		if root.IsSubstring(word) {
+			matching = append(matching, word)
+		}
+	}
+
+	return matching
+}