题目描述:

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:

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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.

实现一个栈的同时能在常数时间内取得栈内的最小值. 使用两个栈即可, 一个栈用来保存元素, 另一个栈用来保存对应元素时栈内的最小值.

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class MinStack {
public:
    vector<int> stack;
    vector<int> min;
    void push(int x) {
        stack.push_back(x);
        if(min.empty() || min.back() > x) min.push_back(x);
        else min.push_back(min.back());
    }

    void pop() {
        stack.pop_back();
        min.pop_back();
    }

    int top() {
        if(stack.empty()) return -1;
        return stack.back();
    }

    int getMin() {
        return min.back();
    }
};