LeetCode in Erlang

20. Valid Parentheses

Easy

Given a string s 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.

Example 1:

Input: s = “()”

Output: true

Example 2:

Input: s = “()[]{}”

Output: true

Example 3:

Input: s = “(]”

Output: false

Example 4:

Input: s = “([)]”

Output: false

Example 5:

Input: s = “{[]}”

Output: true

Constraints:

• 1 <= s.length <= 104
• s consists of parentheses only '()[]{}'.

Solution

%% Check if binary has valid parentheses
-spec is_valid(S :: unicode:unicode_binary()) -> boolean().
is_valid(S) ->
    process_binary(S, []).

%% Process binary with stack, where empty list is empty stack
-spec process_binary(binary(), list()) -> boolean().
process_binary(<<>>, []) -> 
    true;
process_binary(<<>>, _Stack) -> 
    false;
process_binary(<<Char/utf8, Rest/binary>>, Stack) ->
    case Char of
        40 -> process_binary(Rest, [40 | Stack]);  % '('
        91 -> process_binary(Rest, [91 | Stack]);  % '['
        123 -> process_binary(Rest, [123 | Stack]);  % '{'
        41 -> % ')'
            case Stack of
                [40 | NewStack] -> process_binary(Rest, NewStack);
                _ -> false
            end;
        93 -> % ']'
            case Stack of
                [91 | NewStack] -> process_binary(Rest, NewStack);
                _ -> false
            end;
        125 -> % '}'
            case Stack of
                [123 | NewStack] -> process_binary(Rest, NewStack);
                _ -> false
            end;
        _ -> false  % Invalid character
    end.

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