包含min函数的栈

leetcode155

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.

 

class MinStack {
  int min = Integer.MAX_VALUE;
  Stack<Integer> stack = new Stack<>();
  public void push(int x) {
    // only push the old minimum value when the current
    // minimum value changes after pushing the new value x
    if(x <= min){
      stack.push(min);
      min=x;
    }
    stack.push(x);
  }
  public int pop() {
    // if pop operation could result in the changing of the current minimum value,
    // pop twice and change the current minimum value to the last minimum value.
    int var = stack.pop();
    if(var == min) {
      min = stack.pop();
    }
    return var;
  }
  public int top() {
    return stack.peek();
  }
  public int getMin() {
    return min;
  }
}