20.包含min函数的栈

两个栈，一个存所有数据，一个维护栈顶最小。

import java.util.Stack;
public class Solution {
    // 元素栈;
    Stack<Integer> data = new Stack<Integer>();
    // 辅助栈;
    Stack<Integer> min = new Stack<Integer>();
    Integer change = null;
    public void push(int node) {
        if (change != null) {
            if (node <= change) {
                change = node;
                min.push(change);
            }
            data.push(node);
        } else {
            change = node;
            min.push(change);
            data.push(change);
        }
    }
    public void pop() {
        int num1 = data.pop();
        int num2 = min.pop();
        if (num1 != num2) {
            min.push(num2);
        }
    }
    public int top() {
        return data.peek();
    }
    public int min() {
        return min.peek();
    }
}

class Solution {
public:
    stack<int> data;
    stack<int> dataMin;
    void push(int value) {
        if (!dataMin.empty()) {
            int curMin = dataMin.top();
            if (value < curMin) {
                dataMin.push(value);
            }
        } else {
            dataMin.push(value);
        }
        data.push(value);
    }
    void pop() {
        int curTop = data.top();
        data.pop();
        int curMin = dataMin.top();
        dataMin.pop();
        if (curMin != curTop) {
            dataMin.push(curMin);
        }
    }
    int top() {
        return data.top();
    }
    int min() {
        return dataMin.top();
    }
};