[LeetCode] Problem 20 - Valid Parentheses

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

An input string is valid if:

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

Note that an empty string is also considered valid.

Example

No.1

Input: “()”

Output: true

No.2

Input: “()[]{}”

Output: true

No.3

Input: “(]”

Output: false

No.4

Input: “([)]”

Output: false

No.5

Input: “{[]}”

Output: true

Code

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public boolean isValid(String s) {
    Map<Character, Character> map = new HashMap<Character, Character>(){{
        put('(', ')');
        put('[', ']');
        put('{', '}');
    }};

    Stack<Character> stack = new Stack<>();

    for (Character ch : s.toCharArray()) {
        if (map.containsKey(ch))
            stack.push(ch);
        else if (stack.isEmpty() || map.get(stack.pop()) != ch)
            return false;
    }

    return stack.isEmpty();
}