[Design] Add solutions to Max Stack and Design HashMap

Author: soapyigu

Design/DesignHashMap.swift

@@ -0,0 +1,62 @@
+/**
+ * Question Link: https://leetcode.com/problems/design-hashmap/
+ * Primary idea: Use modulo and array of list / linked list to handle handle function and collision.
+ * Time Complexity: O(n), Space Complexity: O(n)
+ */
+
+class MyHashMap {
+    
+    let keySpace = 2069
+    var buckets: [[(Int, Int)]]
+    
+    /** Initialize your data structure here. */
+    init() {
+        buckets = Array(repeating: [(Int, Int)](), count: keySpace)
+    }
+    
+    /** value will always be non-negative. */
+    func put(_ key: Int, _ value: Int) {
+        var bucket = buckets[key % keySpace]
+        
+        if let index = find(key, bucket) {
+            bucket[index].1 = value
+        } else {
+            bucket.append((key, value))
+        }
+        
+        buckets[key % keySpace] = bucket
+    }
+    
+    /** Returns the value to which the specified key is mapped, or -1 if this map contains no mapping for the key */
+    func get(_ key: Int) -> Int {
+        let bucket = buckets[key % keySpace]
+        
+        if let index = find(key, bucket) {
+            return bucket[index].1
+        } else {
+            return -1
+        }
+    }
+    
+    /** Removes the mapping of the specified value key if this map contains a mapping for the key */
+    func remove(_ key: Int) {
+        var bucket = buckets[key % keySpace]
+        
+        guard let index = find(key, bucket) else {
+            return
+        }
+        
+        bucket.swapAt(index, bucket.count - 1)
+        bucket.removeLast()
+        
+        buckets[key % keySpace] = bucket
+    }
+    
+    private func find(_ key: Int, _ bucket: [(Int, Int)]) -> Int? {
+        for (i, pair) in bucket.enumerated() where pair.0 == key {
+            return i
+        }
+        
+        return nil
+    }
+}

README.md

@@ -4,7 +4,7 @@
 ![Leetcode](./logo.png?style=centerme)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 308 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
+[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 310 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
 
 ## Contributors
 
@@ -160,6 +160,7 @@
 | Title | Solution | Difficulty | Time | Space |
 | ----- | -------- | ---------- | ---- | ----- |
 [Min Stack](https://leetcode.com/problems/min-stack/)| [Swift](./Stack/MinStack.swift)| Easy| O(1)| O(n)|
+[Max Stack](https://leetcode.com/problems/max-stack/)| [Swift](./Stack/MaxStack.swift)| Easy| O(n)| O(n)|
 [Valid Parentheses](https://leetcode.com/problems/valid-parentheses/)| [Swift](./Stack/ValidParentheses.swift)| Easy| O(n)| O(n)|
 [Longest Valid Parentheses](https://leetcode.com/problems/longest-valid-parentheses/)| [Swift](./Stack/LongestValidParentheses.swift)| Hard| O(n)| O(n)|
 [Evaluate Reverse Polish Notation](https://leetcode.com/problems/evaluate-reverse-polish-notation/)| [Swift](./Stack/EvaluateReversePolishNotation.swift)| Medium| O(n)| O(n)|
@@ -387,6 +388,7 @@
 | Title | Solution | Difficulty | Time | Space |
 | ----- | -------- | ---------- | ---- | ----- |
 [Shuffle an Array](https://leetcode.com/problems/shuffle-an-array/)| [Swift](./Design/ShuffleAnArray.swift)| Easy| O(n)| O(1)|
+[Design HashMap](https://leetcode.com/problems/design-hashmap/)| [Swift](./Design/DesignHashMap.swift)| Easy| O(n)| O(n)|
 [Design Tic-Tac-Toe](https://leetcode.com/problems/design-tic-tac-toe/)| [Swift](./Design/DesignTicTacToe.swift)| Medium| O(1)| O(n)|
 [Flatten Nested List Iterator](https://leetcode.com/problems/flatten-nested-list-iterator)| [Swift](./Design/FlattenNestedListIterator.swift)| Medium| O(n)| O(n)|
 [Flatten 2D Vector](https://leetcode.com/problems/flatten-2d-vector/)| [Swift](./Design/Vector2D.swift)| Medium | O(n)| O(n)|

Stack/MaxStack.swift

@@ -0,0 +1,54 @@
+/**
+ * Question Link: https://leetcode.com/problems/max-stack/
+ * Primary idea: Use a helper stack to keep track of the max value overall when a new
+ * 				 element is pushed and also updates the helper stack when pop API is called
+ *
+ * Time Complexity: push - O(1), pop - O(1), top - O(1), peekMax - O(1), popMax - O(n) 
+ * Space Complexity: O(n)
+ */
+
+ class MaxStack {
+    
+    var stack: [Int]
+    var maxStack: [Int]
+
+    /** initialize your data structure here. */
+    init() {
+        stack = [Int]()
+        maxStack = [Int]()
+    }
+    
+    func push(_ x: Int) {
+        stack.append(x)
+        maxStack.append(maxStack.isEmpty ? x : max(x, maxStack.last!))
+    }
+    
+    func pop() -> Int {
+        maxStack.removeLast()
+        return stack.removeLast()
+    }
+    
+    func top() -> Int {
+        return stack.last!
+    }
+    
+    func peekMax() -> Int {
+        return maxStack.last!
+    }
+    
+    func popMax() -> Int {
+        let maxVal = peekMax()
+        
+        // remove max from stack
+        var buffer = [Int]()
+        while top() != maxVal {
+            buffer.append(pop())
+        }
+        pop()
+        while !buffer.isEmpty {
+            push(buffer.removeLast())
+        }
+        
+        return maxVal
+    }
+}