Add solution 0820

leetcode/0820.Short-Encoding-of-Words/README.md

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+# [820. Short Encoding of Words](https://leetcode.com/problems/short-encoding-of-words/)
+
+
+## 题目
+
+A **valid encoding** of an array of `words` is any reference string `s` and array of indices `indices` such that:
+
+- `words.length == indices.length`
+- The reference string `s` ends with the `'#'` character.
+- For each index `indices[i]`, the **substring** of `s` starting from `indices[i]` and up to (but not including) the next `'#'` character is equal to `words[i]`.
+
+Given an array of `words`, return *the **length of the shortest reference string*** `s` *possible of any **valid encoding** of* `words`*.*
+
+**Example 1:**
+
+```
+Input: words = ["time", "me", "bell"]
+Output: 10
+Explanation: A valid encoding would be s = "time#bell#" and indices = [0, 2, 5].
+words[0] = "time", the substring of s starting from indices[0] = 0 to the next '#' is underlined in "time#bell#"
+words[1] = "me", the substring of s starting from indices[1] = 2 to the next '#' is underlined in "time#bell#"
+words[2] = "bell", the substring of s starting from indices[2] = 5 to the next '#' is underlined in "time#bell#"
+```
+
+**Example 2:**
+
+```
+Input: words = ["t"]
+Output: 2
+Explanation: A valid encoding would be s = "t#" and indices = [0].
+```
+
+**Constraints:**
+
+- `1 <= words.length <= 2000`
+- `1 <= words[i].length <= 7`
+- `words[i]` consists of only lowercase letters.
+
+## 题目大意
+
+单词数组 words 的 有效编码 由任意助记字符串 s 和下标数组 indices 组成，且满足：
+
+- words.length == indices.length
+- 助记字符串 s 以 '#' 字符结尾
+- 对于每个下标 indices[i] ，s 的一个从 indices[i] 开始、到下一个 '#' 字符结束（但不包括 '#'）的 子字符串 恰好与 words[i] 相等
+
+给你一个单词数组 words ，返回成功对 words 进行编码的最小助记字符串 s 的长度 。
+
+## 解题思路
+
+- 暴力解法。先将所有的单词放入字典中。然后针对字典中的每个单词，逐一从字典中删掉自己的子字符串，这样有相同后缀的字符串被删除了，字典中剩下的都是没有共同前缀的。最终的答案是剩下所有单词用 # 号连接之后的总长度。
+- Trie 解法。构建 Trie 树，相同的后缀会被放到从根到叶子节点中的某个路径中。最后依次遍历一遍所有单词，如果单词最后一个字母是叶子节点，说明这个单词是要选择的，因为它可能是包含了一些单词后缀的最长单词。累加这个单词的长度并再加 1(# 字符的长度)。最终累加出来的长度即为题目所求的答案。
+
+## 代码
+
+```go
+package leetcode
+
+// 解法一 暴力
+func minimumLengthEncoding(words []string) int {
+	res, m := 0, map[string]bool{}
+	for _, w := range words {
+		m[w] = true
+	}
+	for w := range m {
+		for i := 1; i < len(w); i++ {
+			delete(m, w[i:])
+		}
+	}
+	for w := range m {
+		res += len(w) + 1
+	}
+	return res
+}
+
+// 解法二 Trie
+type node struct {
+	value byte
+	sub   []*node
+}
+
+func (t *node) has(b byte) (*node, bool) {
+	if t == nil {
+		return nil, false
+	}
+	for i := range t.sub {
+		if t.sub[i] != nil && t.sub[i].value == b {
+			return t.sub[i], true
+		}
+	}
+	return nil, false
+}
+
+func (t *node) isLeaf() bool {
+	if t == nil {
+		return false
+	}
+	return len(t.sub) == 0
+}
+
+func (t *node) add(s []byte) {
+	now := t
+	for i := len(s) - 1; i > -1; i-- {
+		if v, ok := now.has(s[i]); ok {
+			now = v
+			continue
+		}
+		temp := new(node)
+		temp.value = s[i]
+		now.sub = append(now.sub, temp)
+		now = temp
+	}
+}
+
+func (t *node) endNodeOf(s []byte) *node {
+	now := t
+	for i := len(s) - 1; i > -1; i-- {
+		if v, ok := now.has(s[i]); ok {
+			now = v
+			continue
+		}
+		return nil
+	}
+	return now
+}
+
+func minimumLengthEncoding1(words []string) int {
+	res, tree, m := 0, new(node), make(map[string]bool)
+	for i := range words {
+		if !m[words[i]] {
+			tree.add([]byte(words[i]))
+			m[words[i]] = true
+		}
+	}
+	for s := range m {
+		if tree.endNodeOf([]byte(s)).isLeaf() {
+			res += len(s)
+			res++
+		}
+	}
+	return res
+}
+```