added #155

README.md

@@ -13,6 +13,7 @@ Runtime: 0 ms, faster than 100.00% of Swift online submissions for Valid Parenth
 * [#42 Trapping Rain Water](https://leetcode.com/problems/trapping-rain-water/)
 * [#48 Rotate Image](https://leetcode.com/problems/rotate-image/)
 * [#98 Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)
+* [#155 Min Stack](https://leetcode.com/problems/min-stack/)
 * [#200 Number of Islands](https://leetcode.com/problems/number-of-islands/)
 * [#240 Search a 2D Matrix II](https://leetcode.com/problems/search-a-2d-matrix-ii/)
 * [#297 Serialize and Deserialize Binary Tree](https://leetcode.com/problems/serialize-and-deserialize-binary-tree/)

leetcode-155-min-stack.swift

@@ -0,0 +1,142 @@
+/**
+
+Copyright (c) 2020 David Seek, LLC
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE
+OR OTHER DEALINGS IN THE SOFTWARE.
+
+
+
+
+
+https://leetcode.com/problems/min-stack/
+
+
+Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
+
+push(x) -- Push element x onto stack.
+pop() -- Removes the element on top of the stack.
+top() -- Get the top element.
+getMin() -- Retrieve the minimum element in the stack.
+
+
+Example 1:
+
+Input
+["MinStack","push","push","push","getMin","pop","top","getMin"]
+[[],[-2],[0],[-3],[],[],[],[]]
+
+Output
+[null,null,null,null,-3,null,0,-2]
+
+Explanation
+MinStack minStack = new MinStack();
+minStack.push(-2);
+minStack.push(0);
+minStack.push(-3);
+minStack.getMin(); // return -3
+minStack.pop();
+minStack.top();    // return 0
+minStack.getMin(); // return -2
+
+*/
+
+/**
+Big O Annotation
+Time complexity O(1) all operations are constant.
+Space complexity O(n) where n is the number of elements in the stack.
+*/
+class MinStack {
+
+    /**
+    Min stack holds all min elements
+    with the current lowest min on the top (last)
+    */
+    private var minStack: [Int] = []
+
+    // Array of all elements on the stack
+    private var array: [Int] = []
+
+    // Empty init
+    init() {}
+
+    func push(_ x: Int) {
+
+        // Add element to general stack    
+        array.append(x)
+
+        // Check if we have a last min element
+        guard let last = minStack.last else {
+
+            // If not add the current element
+            minStack.append(x)
+            return 
+        }
+
+        /**
+        Otherwise check if we have a new minimum element.
+
+        Initially I was setting this just to ">"m
+        but I failed a test that expects same minimum
+        elements to also be stored/returned
+        */
+        if last >= x {
+            minStack.append(x)
+        }
+    }
+
+    func pop() {
+
+        // Pop the last element from the general stack
+        let popped = array.popLast()
+
+        /**
+        Check if it's equal to our last element of the 
+        min stack, if so pop it as well
+        */
+        if let last = minStack.last, last == popped {
+            minStack.popLast()
+        }
+    }
+
+    func top() -> Int {
+
+        /**
+        "Peeks" on the last element of the general array.
+
+        There was no requirement for what to return if 
+        there is no last element so I'm returning -1
+        as this follows the requirements of other challenges
+        on leetcode.com.
+        */
+        return array.last ?? -1
+    }
+
+    func getMin() -> Int {
+
+        /**
+        "Peeks" on the last element of the min stack array.
+
+        There was no requirement for what to return if 
+        there is no last element so I'm returning -1
+        as this follows the requirements of other challenges
+        on leetcode.com.
+        */
+        return minStack.last ?? -1
+    }
+}