更新每日一题-20190603 (azl397985856#26)

Author: WendyGeng

daily/2019-06-03.md

@@ -0,0 +1,209 @@
+## 每日一题 - Longest Common Prefix
+
+### 信息卡片
+
+- 时间：2019-06-03
+- 题目链接：https://leetcode.com/problems/longest-common-prefix/
+- tag：`trie` `binary search`
+
+### 题目描述
+
+```
+Write a function to find the longest common prefix string amongst an array of strings.
+
+If there is no common prefix, return an empty string "".
+
+Example 1:
+Input: ["flower","flow","flight"]
+Output: "fl"
+
+Example 2:
+Input: ["dog","racecar","car"]
+Output: ""
+Explanation: There is no common prefix among the input strings.
+
+Note:
+All given inputs are in lowercase letters a-z.
+```
+
+### 参考答案
+
+#### 二分法
+- 找到字符串数组中长度最短字符串
+- longest common prefix 长度范围 0 ~ minLength
+- 运用`binary search`
+
+参考代码
+```javascript
+/*
+ * @lc app=leetcode id=14 lang=javascript
+ *
+ * [14] Longest Common Prefix
+ */
+
+function isCommonPrefix(strs, middle) {
+  const prefix = strs[0].substring(0, middle);
+  for (let i = 1; i < strs.length; i++) {
+    if (!strs[i].startsWith(prefix)) return false;
+  }
+
+  return true;
+}
+/**
+ * @param {string[]} strs
+ * @return {string}
+ */
+var longestCommonPrefix = function(strs) {
+  // trie 解法
+  // 时间复杂度O(m) 空间复杂度O(m * n)
+
+  // tag: 二分法
+  // 时间复杂度 O(n*logm)  空间复杂度O(1)
+  if (strs.length === 0) return "";
+  if (strs.length === 1) return strs[0];
+
+  let minLen = Number.MAX_VALUE;
+
+  for (let i = 0; i < strs.length; i++) {
+    minLen = Math.min(minLen, strs[i].length);
+  }
+
+  let low = 0;
+  let high = minLen;
+
+  while (low <= high) {
+    const middle = (low + high) >> 1;
+    if (isCommonPrefix(strs, middle)) low = middle + 1;
+    else high = middle - 1;
+  }
+
+  return strs[0].substring(0, (low + high) >> 1);
+};
+```
+#### trie树
+以LeetCode另一道题[Implement Trie](https://leetcode.com/problems/implement-trie-prefix-tree/description/)的解法作为本题的参考思路, 具体代码可以自行补充完善
+
+- 建立 Trie
+- 遍历到有一个children有超过一个子元素为止
+
+
+Trie实现参考代码
+```javascript
+/*
+ * @lc app=leetcode id=208 lang=javascript
+ *
+ * [208] Implement Trie (Prefix Tree)
+ *
+ * https://leetcode.com/problems/implement-trie-prefix-tree/description/
+ *
+ * algorithms
+ * Medium (36.93%)
+ * Total Accepted:    172K
+ * Total Submissions: 455.5K
+ * Testcase Example:  '["Trie","insert","search","search","startsWith","insert","search"]\n[[],["apple"],["apple"],["app"],["app"],["app"],["app"]]'
+ *
+ * Implement a trie with insert, search, and startsWith methods.
+ *
+ * Example:
+ *
+ *
+ * Trie trie = new Trie();
+ *
+ * trie.insert("apple");
+ * trie.search("apple");   // returns true
+ * trie.search("app");     // returns false
+ * trie.startsWith("app"); // returns true
+ * trie.insert("app");
+ * trie.search("app");     // returns true
+ *
+ *
+ * Note:
+ *
+ *
+ * You may assume that all inputs are consist of lowercase letters a-z.
+ * All inputs are guaranteed to be non-empty strings.
+ *
+ *
+ */
+function TrieNode(val) {
+  this.val = val;
+  this.children = [];
+  this.isWord = false;
+}
+
+function computeIndex(c) {
+  return c.charCodeAt(0) - "a".charCodeAt(0);
+}
+/**
+ * Initialize your data structure here.
+ */
+var Trie = function() {
+  this.root = new TrieNode(null);
+};
+
+/**
+ * Inserts a word into the trie.
+ * @param {string} word
+ * @return {void}
+ */
+Trie.prototype.insert = function(word) {
+  let ws = this.root;
+  for (let i = 0; i < word.length; i++) {
+    const c = word[i];
+    const current = computeIndex(c);
+    if (!ws.children[current]) {
+      ws.children[current] = new TrieNode(c);
+    }
+    ws = ws.children[current];
+  }
+  ws.isWord = true;
+};
+
+/**
+ * Returns if the word is in the trie.
+ * @param {string} word
+ * @return {boolean}
+ */
+Trie.prototype.search = function(word) {
+  let ws = this.root;
+  for (let i = 0; i < word.length; i++) {
+    const c = word[i];
+    const current = computeIndex(c);
+    if (!ws.children[current]) return false;
+    ws = ws.children[current];
+  }
+  return ws.isWord;
+};
+
+/**
+ * Returns if there is any word in the trie that starts with the given prefix.
+ * @param {string} prefix
+ * @return {boolean}
+ */
+Trie.prototype.startsWith = function(prefix) {
+  let ws = this.root;
+  for (let i = 0; i < prefix.length; i++) {
+    const c = prefix[i];
+    const current = computeIndex(c);
+    if (!ws.children[current]) return false;
+    ws = ws.children[current];
+  }
+  return true;
+};
+
+/**
+ * Your Trie object will be instantiated and called as such:
+ * var obj = new Trie()
+ * obj.insert(word)
+ * var param_2 = obj.search(word)
+ * var param_3 = obj.startsWith(prefix)
+ */
+```
+#### 暴力法
+
+比较常规的一种解法, 大部分人采用的做法, 这里就不再赘述
+
+### 其他优秀解答
+```
+暂无
+```

daily/README.md

@@ -5,9 +5,14 @@
 
 ### 历史汇总
 
+#### [14.Longest-Common-Prefix](./2019-06-03.md)
+
+tag: `trie` `binary search`
+
+时间：2019-06-03
 
 #### [134.Gas Station](./2019-06-04.md)
 
 tag: `Array`
 
-时间: 2019-06-04
+时间: 2019-06-04