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Max Stack

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Description

Design a max stack that supports push, pop, top, peekMax and popMax.

  1. push(x) – Push element x onto stack.
  2. pop() – Remove the element on top of the stack and return it.
  3. top() – Get the element on the top.
  4. peekMax() – Retrieve the maximum element in the stack.
  5. popMax() – Retrieve the maximum element in the stack, and remove it. If you find more than one maximum elements, only remove the top-most one.

Solution

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class MaxStack {
private:
    map<int, stack<list<int>::iterator> > mapping;
    list<int> lis;
public:
    /** initialize your data structure here. */
    MaxStack() {
        
    }
    
    void push(int x) {
        lis.push_front(x);
        mapping[x].push(lis.begin());
    }
    
    int pop() {
        int ret = lis.front();
        mapping[ret].pop();
        if (mapping[ret].empty()) mapping.erase(ret);
        lis.pop_front();
        return ret;
    }
    
    int top() {
        return lis.front();
    }
    
    int peekMax() {
        return mapping.rbegin()->first;
    }
    
    int popMax() {
        auto iter = mapping.rbegin()->second.top();
        int ret = *iter;
        mapping.rbegin()->second.pop();
        if (mapping.rbegin()->second.empty()) 
            mapping.erase(ret);
        lis.erase(iter);
        return ret;
    }
};

/**
 * Your MaxStack object will be instantiated and called as such:
 * MaxStack obj = new MaxStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.peekMax();
 * int param_5 = obj.popMax();
 */