155. Min Stack

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.

解题要点：

此方法构造的stack不是一根标准栈，但是能将最小值一直保持在stack最顶层。

class MinStack {
    Stack<Integer> stack;
    int min = Integer.MAX_VALUE;
    
    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<Integer>();
    }
    
    public void push(int x) {
        if(x <= min){
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }
    
    public void pop() {
        if(stack.pop() == min)
            min = stack.pop();
    }
    
    public int top() {
        return stack.peek();
    }
    
    public int getMin() {
        return min;
    }
}

/**
 * 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();
 */