finish 173-177

Author: BaffinLee

101-200/173. Binary Search Tree Iterator.md

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+# 173. Binary Search Tree Iterator
+
+- Difficulty: Medium.
+- Related Topics: Stack, Tree, Design.
+- Similar Questions: Binary Tree Inorder Traversal, Flatten 2D Vector, Zigzag Iterator, Peeking Iterator, Inorder Successor in BST.
+
+## Problem
+
+Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.
+
+Calling ```next()``` will return the next smallest number in the BST.
+
+*Note:* ```next()``` and ```hasNext()``` should run in average O(1) time and uses O(*h*) memory, where *h* is the height of the tree. 
+
+**Credits:**Special thanks to @ts for adding this problem and creating all test cases.
+
+## Solution
+
+```javascript
+/**
+ * Definition for binary tree
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+
+/**
+ * @constructor
+ * @param {TreeNode} root - root of the binary search tree
+ */
+var BSTIterator = function(root) {
+  this.stack = [];
+  this.pushAll(root);
+};
+
+
+/**
+ * @this BSTIterator
+ * @returns {boolean} - whether we have a next smallest number
+ */
+BSTIterator.prototype.hasNext = function() {
+  return this.stack.length !== 0;
+};
+
+/**
+ * @this BSTIterator
+ * @returns {number} - the next smallest number
+ */
+BSTIterator.prototype.next = function() {
+  var node = this.stack.pop();
+  this.pushAll(node.right);
+  return node.val;
+};
+
+/**
+ * Your BSTIterator will be called like this:
+ * var i = new BSTIterator(root), a = [];
+ * while (i.hasNext()) a.push(i.next());
+*/
+
+BSTIterator.prototype.pushAll = function (node) {
+  while (node) {
+    this.stack.push(node);
+    node = node.left;
+  }
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity :
+* Space complexity :

101-200/174. Dungeon Game.md

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+# 174. Dungeon Game
+
+- Difficulty: Hard.
+- Related Topics: Binary Search, Dynamic Programming.
+- Similar Questions: Unique Paths, Minimum Path Sum, Cherry Pickup.
+
+## Problem
+
+The demons had captured the princess (**P**) and imprisoned her in the bottom-right corner of a dungeon. The dungeon consists of M x N rooms laid out in a 2D grid. Our valiant knight (**K**) was initially positioned in the top-left room and must fight his way through the dungeon to rescue the princess.
+
+The knight has an initial health point represented by a positive integer. If at any point his health point drops to 0 or below, he dies immediately.
+
+Some of the rooms are guarded by demons, so the knight loses health (**negative** integers) upon entering these rooms; other rooms are either empty (**0's**) or contain magic orbs that increase the knight's health (**positive** integers).
+
+In order to reach the princess as quickly as possible, the knight decides to move only rightward or downward in each step.
+
+**Write a function to determine the knight's minimum initial health so that he is able to rescue the princess.**
+
+For example, given the dungeon below, the initial health of the knight must be at least **7** if he follows the optimal path ```RIGHT-> RIGHT -> DOWN -> DOWN```.
+
+|  |  |  |
+| -- | -- | -- |
+| -2 (K) | -3 | 3 |
+| -5 | -10 | 1 |
+| 10 | 30 | -5 (P) |
+
+**Note:**
+
+- The knight's health has no upper bound.
+- Any room can contain threats or power-ups, even the first room the knight enters and the bottom-right room where the princess is imprisoned.
+
+## Solution
+
+```javascript
+/**
+ * @param {number[][]} dungeon
+ * @return {number}
+ */
+var calculateMinimumHP = function(dungeon) {
+  var m = dungeon.length;
+  var n = dungeon[0].length;
+  var dp = Array(m + 1).fill(0).map(_ => Array(n + 1).fill(Number.MAX_SAFE_INTEGER));
+  var tmp = 0;
+  
+  dp[m][ n - 1] = 1;
+  dp[m - 1][n] = 1;
+  
+  for (var i = m - 1; i >= 0; i--) {
+    for (var j = n - 1; j >= 0; j--) {
+      tmp = Math.min(dp[i][j + 1], dp[i + 1][j]) - dungeon[i][j];
+      dp[i][j] = tmp <= 0 ? 1 : tmp;
+    }
+  }
+  
+  return dp[0][0];
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(m*n).
+* Space complexity : O(m*n).

101-200/175. Combine Two Tables.md

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+# 175. Combine Two Tables
+
+- Difficulty: Easy.
+- Related Topics: .
+- Similar Questions: Employee Bonus.
+
+## Problem
+
+Table: ```Person```
+
+```
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| PersonId    | int     |
+| FirstName   | varchar |
+| LastName    | varchar |
++-------------+---------+
+PersonId is the primary key column for this table.
+```
+
+Table: ```Address```
+
+```
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| AddressId   | int     |
+| PersonId    | int     |
+| City        | varchar |
+| State       | varchar |
++-------------+---------+
+AddressId is the primary key column for this table.
+```
+
+Write a SQL query for a report that provides the following information for each person in the Person table, regardless if there is an address for each of those people:
+
+```
+FirstName, LastName, City, State
+```
+
+## Solution
+
+```sql
+# Write your MySQL query statement below
+select a.FirstName, a.LastName, b.City, b.State from Person a left join Address b on a.PersonId = b.PersonId;
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : 
+* Space complexity : 

101-200/176. Second Highest Salary.md

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+# 176. Second Highest Salary
+
+- Difficulty: Easy.
+- Related Topics: .
+- Similar Questions: .
+
+## Problem
+
+Write a SQL query to get the second highest salary from the ```Employee``` table.
+
+```
++----+--------+
+| Id | Salary |
++----+--------+
+| 1  | 100    |
+| 2  | 200    |
+| 3  | 300    |
++----+--------+
+```
+
+For example, given the above Employee table, the query should return ```200``` as the second highest salary. If there is no second highest salary, then the query should return ```null```.
+
+```
++---------------------+
+| SecondHighestSalary |
++---------------------+
+| 200                 |
++---------------------+
+```
+
+## Solution
+
+```javascript
+# Write your MySQL query statement below
+select max(Salary) as SecondHighestSalary from Employee where Salary < (select max(Salary) from Employee)
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity :
+* Space complexity :

101-200/177. Nth Highest Salary.md

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+# 177. Nth Highest Salary
+
+- Difficulty: Medium.
+- Related Topics: .
+- Similar Questions: .
+
+## Problem
+
+Write a SQL query to get the **n**th highest salary from the ```Employee``` table.
+
+```
++----+--------+
+| Id | Salary |
++----+--------+
+| 1  | 100    |
+| 2  | 200    |
+| 3  | 300    |
++----+--------+
+```
+
+For example, given the above Employee table, the **n**th highest salary where **n** = 2 is ```200```. If there is no **n**th highest salary, then the query should return ```null```.
+
+```
++------------------------+
+| getNthHighestSalary(2) |
++------------------------+
+| 200                    |
++------------------------+
+```
+
+## Solution
+
+```sql
+CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
+BEGIN
+  RETURN (
+      # Write your MySQL query statement below.
+      SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT M, 1
+  );
+END
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity :
+* Space complexity :