Valid Parentheses

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.

Solution:

Use a hash table to store each pair of parentheses as a dictionary. As we iterate through each character, if the parentheses is a starting parentheses, put it in a stack; if the parentheses, compare it to the top one in the stack to see if they are a match.

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