# 155. 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:
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); --> Returns -3.
minStack.pop();
minStack.top(); --> Returns 0.
minStack.getMin(); --> Returns -2.
# Solution
Approach 1: Two stacks.
# Code (Python)
Approach 1:
class MinStack:
def __init__(self):
"""
initialize your data structure here.
"""
self.nums = []
self.min_vals = []
def push(self, x: int) -> None:
self.nums.append(x)
if self.min_vals:
self.min_vals.append(min(self.min_vals[-1], x))
else:
self.min_vals.append(x)
def pop(self) -> None:
if self.nums:
self.nums.pop()
self.min_vals.pop()
def top(self) -> int:
if self.nums:
return self.nums[-1]
def getMin(self) -> int:
if self.nums:
return self.min_vals[-1]
# Code (C++)
Approach 1:
class MinStack {
public:
/** initialize your data structure here. */
MinStack() {
}
void push(int x) {
dataStack.push(x);
if (minStack.empty() || x <= minStack.top())
minStack.push(x);
}
void pop() {
if (dataStack.top() == minStack.top())
minStack.pop();
dataStack.pop();
}
int top() {
return dataStack.top();
}
int getMin() {
return minStack.top();
}
private:
stack<int> dataStack;
stack<int> minStack;
};
/**
* Your MinStack object will be instantiated and called as such:
* MinStack obj = new MinStack();
* obj.push(x);
* obj.pop();
* int param_3 = obj.top();
* int param_4 = obj.getMin();
*/