127 added

Author: aQua

Algorithms/0127.word-ladder/README.md

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+# [127. Word Ladder](https://leetcode.com/problems/word-ladder/)
+
+## 题目
+Given two words (beginWord and endWord), and a dictionary's word list, find the length of shortest transformation sequence from beginWord to endWord, such that:
+1. Only one letter can be changed at a time.
+1. Each transformed word must exist in the word list. Note that beginWord is not a transformed word.
+```
+For example,
+Given:
+beginWord = "hit"
+endWord = "cog"
+wordList = ["hot","dot","dog","lot","log","cog"]
+As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
+return its length 5.
+```
+Note:
+1. Return 0 if there is no such transformation sequence.
+1. All words have the same length.
+1. All words contain only lowercase alphabetic characters.
+1. You may assume no duplicates in the word list.
+1. You may assume beginWord and endWord are non-empty and are not the same.
+
+UPDATE (2017/1/20):
+The wordList parameter had been changed to a list of strings (instead of a set of strings). Please reload the code definition to get the latest changes.
+
+## 解题思路
+
+见程序注释

Algorithms/0127.word-ladder/word-ladder.go

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+package Problem0127
+
+func ladderLength(beginWord string, endWord string, wordList []string) int {
+	// 因为 beginWord 不能做 transformed word
+	// 先删掉 words 中的 beginWord，
+	// 可以让后面的 trans 少很多的断头 path，会加快程序。
+	// 也更符合题意
+	// 删除下面这句，程序也能 accepted，但是会从 269ms 减慢到 319ms
+	words = deleteBeginWord(words, beginWord)
+
+	// trans 用来查找 k->？
+	trans := map[string][]string{}
+	// isTransedEndWord 用于在生成 trans 的过程中标记，存在 ->endWord 的转换关系
+	// 用于提前结束
+	isTransedEndWord := false
+	// cnt 用于记录生成trans的迭代次数
+	// 其实也是最短路径的长度
+	cnt := 1
+	var bfs func([]string, []string)
+	// 使用 bfs 方法，递归地生成 trans
+	bfs = func(words, nodes []string) {
+		cnt++
+		// words 中的 w
+		//     与 nodes 中的 n ，可以实现 n->w 的转换，
+		//       则，w 会被放入 newNodes
+		//     否则，w 会被放入 newWords
+		newWords := make([]string, 0, len(words))
+		newNodes := make([]string, 0, len(words))
+		for _, w := range words {
+			isTransed := false
+			for _, n := range nodes {
+				if isTransable(n, w) {
+					trans[n] = append(trans[n], w)
+					isTransed = true
+				}
+			}
+
+			if isTransed {
+				newNodes = append(newNodes, w)
+				if w == endWord {
+					isTransedEndWord = true
+				}
+			} else {
+				newWords = append(newWords, w)
+			}
+		}
+
+		if isTransedEndWord || // 转换到了 endWord 说明已经找到了所有的最短路径
+			len(newWords) == 0 || // words 的所有单词都已经可以从 beginWord trans 到
+			len(newNodes) == 0 { // newWords 中单词，是 beginWord 无法 trans 到的
+			return
+		}
+
+		// 上面没有 return，要继续完善 trans
+		bfs(newWords, newNodes)
+	}
+
+	// 第一代 nodes 含有且仅含有 beginWord
+	nodes := []string{beginWord}
+	bfs(words, nodes)
+
+	res := [][]string{}
+	if !isTransedEndWord {
+		// beginWord 无法 trans 到 endWord
+		return res
+	}
+
+	path := make([]string, cnt)
+	path[0] = beginWord
+
+	var dfs func(int)
+	// 使用 dfs 方法，生成最短路径
+	dfs = func(idx int) {
+		if idx == cnt {
+			// path 已经填充完毕
+			if path[idx-1] == endWord {
+				// 最后一个单词是 endWord，说明这是一条最短路径
+				res = append(res, deepCopy(path))
+			}
+			return
+		}
+
+		prev := path[idx-1]
+		for _, w := range trans[prev] {
+			// 利用 prev->w 填充 path[idx]
+			path[idx] = w
+			dfs(idx + 1)
+		}
+	}
+
+	dfs(1)
+
+	return res
+}
+
+func deepCopy(src []string) []string {
+	temp := make([]string, len(src))
+	copy(temp, src)
+	return temp
+}
+
+// 题目中说了，words 中没有重复的单词，
+// 所以，beginWord 最多出现一次
+func deleteBeginWord(words []string, beginWord string) []string {
+	i := 0
+	for ; i < len(words); i++ {
+		if words[i] == beginWord {
+			break
+		}
+	}
+
+	if i == len(words) {
+		return words
+	}
+
+	return append(words[:i], words[i+1:]...)
+}
+
+func isTransable(a, b string) bool {
+	// onceAgain == true 说明已经出现过不同的字符了
+	onceAgain := false
+	for i := range a {
+		if a[i] != b[i] {
+			if onceAgain {
+				return false
+			}
+			onceAgain = true
+		}
+	}
+
+	return true
+}

Algorithms/0127.word-ladder/word-ladder_test.go

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+package Problem0127
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_Problem0127(t *testing.T) {
+	ast := assert.New(t)
+
+	// tcs is testcase slice
+	tcs := []struct {
+		beginWord string
+		endWord   string
+		wordList  []string
+		ans       int
+	}{
+
+		{
+			"hit",
+			"cog",
+			[]string{"hot", "dot", "dog", "lot", "log"},
+			0,
+		},
+
+		{
+			"hot",
+			"dog",
+			[]string{"hot", "dog", "dot"},
+			3,
+		},
+
+		{
+			"hit",
+			"cog",
+			[]string{"hot", "dot", "dog", "lot", "log", "cog"},
+			5,
+		},
+
+		// 可以多个 testcase
+	}
+
+	for _, tc := range tcs {
+		fmt.Printf("~~%v~~\n", tc)
+
+		ast.Equal(tc.ans, ladderLength(tc.beginWord, tc.endWord, tc.wordList), "输入:%v", tc)
+	}
+}