[LeetCode] 155. Min Stack

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.
Solution:

建立一个辅助栈,存储当前的最小值。

running time: O(n)

space: O(n)

class MinStack:
    # @param x, an integer
    # @return an integer
    
    def __init__(self):
        self.s = []
        self.hs = [] #create a help stack.
        
    def push(self, x):
        if len(self.s) == 0 or x <= self.hs[-1]:
            self.hs.append(x)
        self.s.append(x)
        

    # @return nothing
    def pop(self):
        if self.s[-1] == self.hs[-1]:
            self.hs.pop()
        self.s.pop()
        
        

    # @return an integer
    def top(self):
        if len(self.s) != 0:
            return self.s[-1]

    # @return an integer
    def getMin(self):
        if len(self.hs) != 0:
            return self.hs[-1]
        


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