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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

import scala.collection.mutable.Stack

object Solution {
    def isValid(s: String): Boolean = {
        val stack = Stack[Char]()

        for (i <- 0 until s.length) {
            val c = s.charAt(i)
            if (c == '(' || c == '[' || c == '{') {
                stack.push(c)
            } else if (c == ')' && stack.nonEmpty && stack.top == '(') {
                stack.pop()
            } else if (c == '}' && stack.nonEmpty && stack.top == '{') {
                stack.pop()
            } else if (c == ']' && stack.nonEmpty && stack.top == '[') {
                stack.pop()
            } else {
                return false
            }
        }

        stack.isEmpty
    }
}

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