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 1:
Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]
Output
[null,null,null,null,-3,null,0,-2]
Explanation:
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top(); // return 0
minStack.getMin(); // return -2
Constraints:
- Methods
pop
,top
andgetMin
operations will always be called on non-empty stacks.
题意:设计一个支持获取最小值的栈,
思路:可以用数组或链表,从最底层搭建;这里我使用的是STL的 stack
。
除了存储数值的栈外,还定义一个存储最小值的栈——如果新进入的值小于等于栈顶元素,则入栈,此时栈顶元素为当前的最小值;getMin
总是取最小栈的栈顶元素;如果元素出栈,发现它是最小栈的栈顶元素,则最小栈也要出栈一次。
代码:
class MinStack {
public:
/** initialize your data structure here. */
stack<int> minVal, st;
MinStack() { }
void push(int x) {
if (minVal.empty()) minVal.push(x);
else if (x <= minVal.top()) minVal.push(x);
st.push(x);
}
void pop() {
if (!st.empty()) {
if (minVal.top() == st.top()) minVal.pop();
st.pop();
}
}
int top() { return st.top(); }
int getMin() { return minVal.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();
*/
效率:
执行用时:40 ms, 在所有 C++ 提交中击败了89.09% 的用户
内存消耗:14.5 MB, 在所有 C++ 提交中击败了100.00% 的用户