add new solutions

solutions/1/q127/solution.go

@@ -0,0 +1,63 @@
+package q127
+
+// Ideas for improvement: use a hashmap to index strings instead of an O(n^2)
+// comparison and neighbor matrix; use bidirectional BFS.
+
+func reachable(a, b string) bool {
+	diff := false
+	for i := range len(a) {
+		if a[i] != b[i] {
+			if diff {
+				return false
+			}
+			diff = true
+		}
+	}
+	return diff // exactly one different char
+}
+
+func ladderLength(beginWord string, endWord string, wordList []string) int {
+	target := -1
+	neighbors := make([][]int, len(wordList))
+
+	for i := range wordList {
+		if target == -1 && wordList[i] == endWord {
+			target = i
+		}
+		for j := i + 1; j < len(wordList); j++ {
+			if reachable(wordList[i], wordList[j]) {
+				neighbors[i] = append(neighbors[i], j)
+				neighbors[j] = append(neighbors[j], i)
+			}
+		}
+	}
+	if target == -1 {
+		return 0
+	}
+
+	seen := make([]bool, len(wordList))
+	queue := make([][2]int, 0, len(wordList)) // index, steps
+	for i := range wordList {
+		if reachable(beginWord, wordList[i]) {
+			seen[i] = true
+			queue = append(queue, [2]int{i, 2})
+		}
+	}
+	for ; len(queue) > 0; queue = queue[1:] {
+		id, step := queue[0][0], queue[0][1]
+		if id == target {
+			return step
+		}
+
+		for _, n := range neighbors[id] {
+			if !seen[n] {
+				seen[n] = true
+				queue = append(queue, [2]int{n, step + 1})
+			}
+		}
+	}
+
+	return 0
+}
+
+var _ = ladderLength