20. Valid Parentheses (Easy)

https://leetcode.com/problems/valid-parentheses/

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

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

Solutions

class Solution {

    // The key idea is to employ stack to solve this problem.

    public boolean isValid(String str) {
        if (str == null) {
            return true;
        }

        Set<Character> openP = new HashSet();

        openP.add('(');
        openP.add('{');
        openP.add('<');
        openP.add('[');

        Set<Character> closeP = new HashSet();

        closeP.add(')');
        closeP.add('}');
        closeP.add('>');
        closeP.add(']');

        Map<Character, Character> pairs = new HashMap<>();

        pairs.put(')', '(');
        pairs.put('}', '{');
        pairs.put('>', '<');
        pairs.put(']', '[');

        Stack<Character> stack = new Stack<>();
        for (int i = 0, l = str.length(); i < l; i++) {

            // must be Object type, rather than primary type
            Character c = str.charAt(i);

            if (openP.contains(c)) {
                stack.push(c);
                continue;
            }

            if (closeP.contains(c)) {
                if (stack.size() == 0) {
                    return false;
                }

                if (pairs.get(c) != stack.pop()) {
                    return false;
                }
            }
        }

        return stack.isEmpty();
    }
}

Incorrect Solutions

References