155.最小栈
题目
设计一个支持 push ,pop ,top 操作,并能在常数时间内检索到最小元素的栈。
实现 MinStack 类:
MinStack()初始化堆栈对象。void push(int val)将元素val推入堆栈。void pop()删除堆栈顶部的元素。int top()获取堆栈顶部的元素。int getMin()获取堆栈中的最小元素。
示例 1:
输入:
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]
输出:
[null,null,null,null,-3,null,0,-2]
解释:
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); --> 返回 -3.
minStack.pop();
minStack.top(); --> 返回 0.
minStack.getMin(); --> 返回 -2.
提示:
-231 <= val <= 231 - 1pop、top和getMin操作总是在 非空栈 上调用push,pop,top, andgetMin最多被调用3 * 104次
思路
用栈存储正常的值,再用一个栈从大到小存储最小值
代码
java
class MinStack {
private Stack<Integer> stack;
private Stack<Integer> minStack;
public MinStack() {
this.stack = new Stack<>();
this.minStack = new Stack<>();
}
public void push(int val) {
stack.push(val);
if (!minStack.isEmpty()) {
int top = minStack.peek();
if (top >= val) minStack.push(val);
} else minStack.push(val);
}
public void pop() {
int pop = stack.pop();
int top = minStack.peek();
if (pop == top) minStack.pop();
}
public int top() {
return stack.peek();
}
public int getMin() {
return minStack.peek();
}
}