leetcode 20. Valid Parentheses

题目

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

解析

class Solution {
    public boolean isValid(String s) {
        Deque<Character> stack = new ArrayDeque<>();
        char[] ch = s.toCharArray();
        int n = ch.length;
        for (int i = 0; i < n; i++) {
            char c = ch[i];
            if (c == '{' || c == '[' || c == '(') stack.push(c);
            else {
                if (stack.size() == 0) return false;
                char cc = stack.pop();
                if (cc == '{' && c != '}') return false;
                if (cc == '[' && c != ']') return false;
                if (cc == '(' && c != ')') return false;
            }
        }
        if (!stack.isEmpty()) return false;
        return true;
    }
}

public boolean isValid(String s) {
	Stack<Character> stack = new Stack<Character>();
	for (char c : s.toCharArray()) {
		if (c == '(')
			stack.push(')');
		else if (c == '{')
			stack.push('}');
		else if (c == '[')
			stack.push(']');
		else if (stack.isEmpty() || stack.pop() != c)
			return false;
	}
	return stack.isEmpty();
}