Saturday, June 1, 2019

LeetCode [716] Max Stack

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.
Example 1:
MaxStack stack = new MaxStack();
stack.push(5); 
stack.push(1);
stack.push(5);
stack.top(); -> 5
stack.popMax(); -> 5
stack.top(); -> 1
stack.peekMax(); -> 5
stack.pop(); -> 1
stack.top(); -> 5
Note:
  1. -1e7 <= x <= 1e7
  2. Number of operations won't exceed 10000.
  3. The last four operations won't be called when stack is empty.
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class MaxStack {
    stack<int> stk, stkM;
public:
    /** initialize your data structure here. */
    MaxStack() {
        
    }
    
    void push(int x) {
        stk.push(x);
        if(stkM.empty() || x>=stkM.top()) stkM.push(x);
    }
    
    int pop() {
        int v = stk.top();
        stk.pop();
        if(stkM.top()==v) stkM.pop();
        return v;
    }
    
    int top() {
        return stk.top();
    }
    
    int peekMax() {
        return stkM.top();
    }
    
    int popMax() {
        int v = stkM.top();
        stkM.pop();
        if(v==stk.top()){
            stk.pop();
        }else{
            stack<int> tmp;
            while(!stk.empty() && stk.top()!=v){
                tmp.push(stk.top());
                stk.pop();
            }
            stk.pop();
            while(!tmp.empty()){
                stk.push(tmp.top());
                if(stkM.empty() || tmp.top()>=stkM.top()) stkM.push(tmp.top());
                tmp.pop();
            }
        }
        
        return v;
    }
};

/**
 * 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();
 */

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