栈

栈(LIFO)

学pwn的看到栈真是惬意啊

不对…

20.有效的括号

解：

后进先出

class Solution {
public:
    bool isValid(string s) {
        stack<char> stk;
        for(char c : s) {
            if(c == '(' || c == '[' || c == '{'){
                stk.push(c);
            }else if(c == ')' && !stk.empty() && stk.top() == '('){
                stk.pop();
            }else if(c == ']' && !stk.empty() && stk.top() == '['){
                stk.pop();
            }else if(c == '}' && !stk.empty() && stk.top() == '{'){
                stk.pop();
            }else{
                return false;
            }
        }
        return stk.empty();
    }
};

155.最小栈

解:

class MinStack {
private:
    stack<int> st;
    stack<int> minSt; 
public:
    MinStack() {}
    
    void push(int val) {
        st.push(val);
        if (minSt.empty() || val <= minSt.top()) {
            minSt.push(val);
        }
    }
    
    void pop() {
        if(st.top() == minSt.top()) {
            minSt.pop();
        }
        st.pop();
    }
    
    int top() {
        return st.top();
    }
    
    int getMin() {
        return minSt.top();
    }
};

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack* obj = new MinStack();
 * obj->push(val);
 * obj->pop();
 * int param_3 = obj->top();
 * int param_4 = obj->getMin();
 */