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.

C++程式碼:

class MinStack {
public:
    /** initialize your data structure here. */
 

    void push(int x) {
      v.push_back(x);
    }
    void pop() {
      v.pop_back();
    }
    int top() {
      return v.back();
    }
    int getMin() {
      int len = v.size();
      int min = v[0];
      for(int i = 1; i < len;i++) {
        if(v[i] < min) min = v[i];
      }
      return
 
 min;
    }
private:
  vector<int> v;
};

思路1:

• 簡單的棧的應用, 遍歷一下vector元素即可

思路2:

• 一般思路是用一個變數min記錄當前最小, 但如果當前最小的元素出棧, 沒辦法記錄min 為出棧後第一小的元素(即出棧前第二小的元素)
• 使用另一個vector min記錄最小元素, 棧頂元素為當前最小
• 當vector出棧一個元素, 如果為當前最小, min 出棧, 此次 min的棧頂仍然為當前最小元素

C++程式碼:

class MinStack {
public:
    /** initialize your data structure here. */
    void push(int x) {
      v.push_back(x);
      if(min.empty() || x <= min.back()) min.push_back(x);
    }
    void pop() {
      if(v.back() == min.back()) min.pop_back();
      v.pop_back();
    }
    int top() {
      return v.back();
    }
    int getMin() {
      return min.back();
    }
private:
  vector<int> v;
  vector<int> min;;
};