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84 lines
2.2 KiB
Plaintext
84 lines
2.2 KiB
Plaintext
---
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title: '155. Min Stack'
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description: Design a stack that supports push, pop, top, and retrieving the minimum element in constant time
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icon: dot
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sidebar:
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label: 'Min Stack'
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badge: 'Medium'
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---
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<Badge variant="accent">Stack</Badge>
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::::warning
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You must implement a solution with O(1) time complexity for each function.
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::::
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### Example 1:
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- Input: ``
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- Output: ``
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- Explanation: 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`
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### Constraints:
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- `-2^31 <= val <= 2^31 - 1`
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- Methods pop, top and getMin operations will always be called on non-empty stacks.
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- At most 3 * 10^4 calls will be made to push, pop, top, and getMin.
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## Approach
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```mermaid
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flowchart TD
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subgraph state["two parallel stacks — same depth, always"]
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A["stack — the values"]
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B["minStack — the minimum as of that depth"]
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end
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state --> P
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subgraph P["push(value)"]
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P1["stack.append(value)"] --> P2["minStack.append(min(value, minStack[-1]))"]
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end
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state --> O
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subgraph O["pop()"]
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O1["pop both stacks together"]
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end
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state --> R
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subgraph R["top() / getMin() — O(1)"]
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R1(["stack[-1] / minStack[-1]"])
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end
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```
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## Solution
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```py
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class MinStack:
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def __init__(self):
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self.stack = []
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self.minStack = []
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def push(self, value: int) -> None:
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self.stack.append(value)
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value = min(value, self.minStack[-1] if self.minStack else value)
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self.minStack.append(value)
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def pop(self) -> None:
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self.stack.pop()
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self.minStack.pop()
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def top(self) -> int:
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return self.stack[-1]
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def getMin(self) -> int:
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return self.minStack[-1]
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# Your MinStack object will be instantiated and called as such:
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# obj = MinStack()
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# obj.push(value)
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# obj.pop()
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# param_3 = obj.top()
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# param_4 = obj.getMin()
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```
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## Explanation
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<YouTube id="qkLl7nAwDPo" title="Design Min Stack - Amazon Interview Question - Leetcode 155 - Python" />
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