LeetCode - E- Min Stack

題意

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:

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.

解法

兩個棧，一個棧存原始數據，一個棧始終保留當前最小元素

實現

class MinStack {
private:
stack<int> oriStack;
stack<int> minStack;
public:
    /** initialize your data structure here. */
    MinStack() {

    }

    void push(int x) {
        oriStack.push(x);
        if(minStack.empty()) minStack.push(x);
        else if(minStack.top() < x) minStack.push(minStack.top());
        else minStack.push(x);
    }

    void pop() {
        if(oriStack.empty()) return;
        oriStack.pop();
        minStack.pop();
    }

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

    int getMin() {
        if(oriStack.empty()) return -1;
        return minStack.top();
    }
};

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */