【LeetCode】155. Min Stack

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155. Min Stack

Description：
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
Difficulty：Easy
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.

方法: 双数组

• Time complexity : $O\left ( n\right )$
• Space complexity : $O\left ( 1\right )$
  思路：
  一个数组存储栈元素，另一个存储最小元素，可以将vector换成两个stack

class MinStack {
private:
    vector<int> list;
    vector<int> min_list;
public:
	/** initialize your data structure here. */
	void push(int x) {
		list.push_back(x);
		if(min_list.size() == 0)
            min_list.push_back(x);
        else
            min_list.push_back(min(min_list.back(), x));
	}

	void pop() {
		if (list.size() == 0) return;
		list.pop_back();
		min_list.pop_back();
	}

	int top() {
		if (list.size() == 0) return NULL;
		return list.back();
	}

	int getMin() {
		if (min_list.size() == 0) return NULL;
		return min_list.back();
	}
};