栈

有效括号

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class Solution {
    public boolean isValid(String s) {
        if (s == null || s.length() == 0){
            return false;
        }
        Stack<Character> stack = new Stack<>();
        for(char c : s.toCharArray()){
            if (c == '(' || c == '[' || c == '{'){
                stack.push(c);
            //这里容易漏掉，需注意
            } else if(stack.empty() && (c == ')' || c == ']'||c == '}')){
                return false;
            }else{
                char a = stack.pop();
                if (a == '(' && c != ')'){
                    return false;
                }
                if (a == '[' && c != ']'){
                    return false;
                }
                if (a == '{' && c != '}'){
                    return false;
                } 
            }
        }
        return stack.empty();
    }
}

最小栈

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class MinStack {
    Stack<Integer> stack;
    Stack<Integer> minStack;

    public MinStack() {
        stack = new Stack();
        minStack = new Stack();

    }
    
    public void push(int val) {
        stack.push(val);
        if(minStack.empty() || minStack.peek() > val){
            minStack.push(val);
        } else {
            //注意这里，如果不push自身的话，pop会空栈异常
            minStack.push(minStack.peek());
        }
    }
    
    public void pop() {
        stack.pop();
        minStack.pop();
    }
    
    public int top() {
        return stack.peek();
    }
    
    public int getMin() {
        return minStack.peek();
    }
}