LC20. 有效的括号

class Solution {
    public boolean isValid(String s) {
        //使用栈
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i ++){
            if (s.charAt(i) == '(')
                stack.push(s.charAt(i));
            if (s.charAt(i) == '[')
                stack.push(s.charAt(i));
            if (s.charAt(i) == '{')
                stack.push(s.charAt(i));
            if ( s.charAt(i) == ')'){
                if (!stack.empty() && stack.peek() == '(')
                    stack.pop();
                else return false;
            }
            if (s.charAt(i) == ']'){
                if (!stack.empty() && stack.peek() == '[')
                    stack.pop();
                else return false;
            }
            if (s.charAt(i) == '}'){
                if (!stack.empty() && stack.peek() == '{')
                    stack.pop();
                else return false;
            }
        }
        if (stack.empty())
            return true;
        return false;
    }
}

LC155. 最小栈

自己写的竟然只击败了7%…

class MinStack {
    Stack<Integer> s = new Stack<>();
    ArrayList<Integer> minList = new ArrayList<>();
    int idx = -1;
    int min = 2147483647;
    /** initialize your data structure here. */
    public MinStack() {

    }

    public void push(int val) {
        //比较的时候要比较整个minList的最小值，在常数范围求最小
        min = Math.min(val, (idx >= 0)?minList.get(idx):min);
        minList.add(min);
        idx++;
        s.push(val);
    }

    public void pop() {
        min = 2147483647;
        if (idx >= 0)
        {
            minList.remove(idx);
            idx--;
        }
        s.pop();
    }

    public int top() {
        return s.peek();
    }

    public int getMin() {
        System.out.println(idx);
        return minList.get(idx);
    }
}

/**
 * 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();
 */

未完待续…`