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Author: BaffinLee

201-300/208. Implement Trie (Prefix Tree).md

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+# 208. Implement Trie (Prefix Tree)
+
+- Difficulty: Medium.
+- Related Topics: Design, Trie.
+- Similar Questions: Add and Search Word - Data structure design, Design Search Autocomplete System, Replace Words, Implement Magic Dictionary.
+
+## Problem
+
+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.
+
+## Solution
+
+```javascript
+var Node = function () {
+  this.children = {};
+  this.isWord = false;
+};
+
+/**
+ * Initialize your data structure here.
+ */
+var Trie = function() {
+  this.root = new Node();
+};
+
+/**
+ * Inserts a word into the trie. 
+ * @param {string} word
+ * @return {void}
+ */
+Trie.prototype.insert = function(word) {
+  var len = word.length;
+  var node = this.root;
+  var char = 0;
+  for (var i = 0; i < len; i++) {
+    char = word[i];
+    if (!node[char]) node[char] = new Node();
+    node = node[char];
+  }
+  node.isWord = true;
+};
+
+/**
+ * Returns if the word is in the trie. 
+ * @param {string} word
+ * @return {boolean}
+ */
+Trie.prototype.search = function(word) {
+  var len = word.length;
+  var node = this.root;
+  var char = 0;
+  for (var i = 0; i < len; i++) {
+    char = word[i];
+    if (!node[char]) return false;
+    node = node[char];
+  }
+  return node.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) {
+  var len = prefix.length;
+  var node = this.root;
+  var char = 0;
+  for (var i = 0; i < len; i++) {
+    char = prefix[i];
+    if (!node[char]) return false;
+    node = node[char];
+  }
+  return true;
+};
+
+/** 
+ * Your Trie object will be instantiated and called as such:
+ * var obj = Object.create(Trie).createNew()
+ * obj.insert(word)
+ * var param_2 = obj.search(word)
+ * var param_3 = obj.startsWith(prefix)
+ */
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(h).
+* Space complexity : O(h).

201-300/215. Kth Largest Element in an Array.md

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+# 215. Kth Largest Element in an Array
+
+- Difficulty: Medium.
+- Related Topics: Divide and Conquer, Heap.
+- Similar Questions: Wiggle Sort II, Top K Frequent Elements, Third Maximum Number, Kth Largest Element in a Stream.
+
+## Problem
+
+Find the **k**th largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.
+
+**Example 1:**
+
+```
+Input: [3,2,1,5,6,4] and k = 2
+Output: 5
+```
+
+**Example 2:**
+
+```
+Input: [3,2,3,1,2,4,5,5,6] and k = 4
+Output: 4
+```
+
+**Note: **
+You may assume k is always valid, 1 ≤ k ≤ array's length.
+
+## Solution
+
+```javascript
+/**
+ * @param {number[]} nums
+ * @param {number} k
+ * @return {number}
+ */
+var findKthLargest = function(nums, k) {
+  return quickSelect(nums, 0, nums.length - 1, k);
+};
+
+var quickSelect = function (nums, left, right, k) {
+  var le = left;
+  var ri = right;
+  var mid = nums[right];
+  while (le < ri) {
+    if (nums[le++] > mid) swap(nums, --le, --ri);
+  }
+  swap(nums, le, right);
+  var len = right - le;
+  if (len === k - 1) return nums[le];
+  else if (len < k - 1) return quickSelect(nums, left, le - 1, k - len - 1);
+  else return quickSelect(nums, le + 1, right, k);
+};
+
+var swap = function (nums, i, j) {
+  var tmp = nums[i];
+  nums[i] = nums[j];
+  nums[j] = tmp;
+}
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(1).