#22 Medium Time O(1) Space O(n) Animated

Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element.

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1 Predict
2 Trace
3 Explain
4 Implement

Pattern LIFO state for unresolved work

Keep only items whose matching decision has not been made yet.

Approach: Paired Value & Minimum Stack

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Core Insight

Keep only items whose matching decision has not been made yet.

Common Pitfall

Check that the stack is non-empty before reading its top.

Description

Design a stack that supports push, pop, top, and retrieving the minimum element.

Constraints

Time Complexity: O(1)
Space Complexity: O(n)

Solution — Paired Value & Minimum Stack

TimeO(1) SpaceO(n)

Implementation Plan

Translate the invariant into these small, testable moves.

Read one item at a time.
Push unresolved work.
Match or remove work when its partner arrives.
Validate the final stack state.

class MinStack:
    def __init__(self):
        self.stack = []

    def push(self, val):
        current_min = min(val, self.getMin()) if self.stack else val
        self.stack.append((val, current_min))

    def pop(self):
        self.stack.pop()

    def top(self):
        return self.stack[-1][0]

    def getMin(self):
        return self.stack[-1][1]

Valid Parentheses

Evaluate Reverse Polish Notation

Min Stack

Steps

Description

Constraints

Tags

Solution — Paired Value & Minimum Stack