Completed Problem 1

Exercise_2.py

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+
+class MinStack:
+    '''
+    Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
+
+    Implement the MinStack class:
+    MinStack() initializes the stack object.
+    void push(int val) pushes the element val onto the stack.
+    void pop() removes the element on the top of the stack.
+    int top() gets the top element of the stack.
+    int getMin() retrieves the minimum element in the stack.
+
+    You must implement a solution with O(1) time complexity for each function.
+
+    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
+
+    Solution
+        1. Keep two separate stacks, main stack and min stack
+           For each new item pushed to main stack, compute a running min which is the min of all numbers from bottom to top of main stack. The running min is computed by "curr_min". "curr_min" is initialized to
+           infinity.
+
+           For each new item pushed to the stack, "curr_min" is computed as,
+                curr_min = min(curr_min, new item)
+           Push new item to main stack
+           Push curr_min to min stack
+
+           For each pop,
+           Pop from main stack
+           Pop from min stack
+
+           Example:
+                push(-2), push(0), push(-3)
+                    mainStack  = [-2, 0, -3]
+                    minStack = [-2, -2, -3]
+                    getMin = -3 (minStack[-1])
+                pop()
+                    mainStack = [-2, 0]
+                    minStack = [-2, -2]
+                    top mainStack = 0
+                    getMin = -2 (minStack[-1])
+
+           Time: O(1), Space: O(N)
+    '''
+    def __init__(self):
+        '''
+            init stack
+
+            Time: O(1), Space: O(1)
+        '''
+        self.main_stack = []
+        self.min = float('inf')
+        self.min_stack = [self.min]
+        self.length = 0
+
+    def push(self, val: int) -> None:
+        '''
+            push item to stack
+
+            Time: O(1), Space: O(1)
+        '''
+        self.main_stack.append(val)
+        self.min  = min(self.min, val)
+        self.min_stack.append(self.min)
+        self.length += 1
+        print(f"push {val}: main stack = {self.main_stack}, min stack = {self.min_stack}, stack len = {self.length}")
+
+
+    def pop(self) -> None:
+        '''
+            pop item from stack
+
+            Time: O(1), Space: O(1)
+        '''
+        if self.length > 0:
+            val = self.main_stack.pop()
+            self.min_stack.pop()
+            self.min = self.min_stack[-1]
+            self.length -= 1
+            print(f"popped {val}: main stack = {self.main_stack}, min stack = {self.min_stack}, stack len = {self.length}")
+
+        else:
+            print(f"Cannot pop, Empty stack")
+
+
+    def top(self) -> int:
+        '''
+            top item to stack
+
+            Time: O(1), Space: O(1)
+        '''
+        val = None
+        if self.length > 0:
+          val = self.main_stack[-1]
+        print(f"top: main stack = {val}")
+        return val
+
+    def getMin(self) -> int:
+        '''
+            get min value of stack
+
+            Time: O(1), Space: O(1)
+        '''
+        print(f"getMin(): {self.min}")
+        return self.min
+
+def run_min_stack():
+    print(f"\n --- MinStack --- ")
+    minStack = MinStack()
+    # minStack.push(-2)
+    # minStack.push(0)
+    # minStack.push(-3)
+    # minStack.getMin() # return -3
+    # minStack.pop()
+    # minStack.top()    # return 0
+    # minStack.getMin() # return -2
+    # minStack.pop()
+    # minStack.getMin() # return -2
+
+    minStack.push(5)
+    minStack.getMin() # 5
+    minStack.push(9)
+    minStack.getMin() # 5
+    minStack.push(4)
+    minStack.getMin() # 4
+    minStack.pop()
+    minStack.getMin() # 5
+    minStack.push(12)
+    minStack.pop()
+    minStack.getMin() # 5
+    minStack.push(3)
+    minStack.getMin() # 3
+    minStack.pop()
+    minStack.getMin() # 5
+    minStack.pop()
+    minStack.getMin() # 5
+    minStack.pop()
+    minStack.getMin() # inf
+
+run_min_stack()