155. Min Stack

Maintain two stacks. One is the normal stack and the other stores the minimum number so far. These two stacks should always have the same size. When I revisit this problem, I made a mistake in line 11.

1:  class MinStack {  
2:  private:  
3:    stack<int> stk;  
4:    stack<int> minStk;  
5:  public:  
6:    /** initialize your data structure here. */  
7:    MinStack() {  
8:    }  
9:    void push(int x) {  
10:      stk.push(x);  
11:      if (!minStk.empty()) {  
12:        minStk.push(x < minStk.top() ? x : minStk.top());  
13:      } else {  
14:        minStk.push(x);  
15:      }  
16:    }  
17:    void pop() {  
18:      if (!stk.empty()) {  
19:        stk.pop();  
20:        minStk.pop();  
21:      }  
22:    }  
23:    int top() {  
24:      if (!stk.empty()) {  
25:        return stk.top();  
26:      } else {  
27:        return -1;  
28:      }  
29:    }  
30:    int getMin() {  
31:      if (!minStk.empty()) {  
32:        return minStk.top();  
33:      } else {  
34:        return -1;  
35:      }  
36:    }  
37:  };  
38:  /**  
39:   * Your MinStack object will be instantiated and called as such:  
40:   * MinStack obj = new MinStack();  
41:   * obj.push(x);  
42:   * obj.pop();  
43:   * int param_3 = obj.top();  
44:   * int param_4 = obj.getMin();  
45:   */