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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

function isValid(s: string): boolean {
    const stack: string[] = []
    for (let i = 0; i < s.length; i++) {
        const c = s.charAt(i)
        if (c === '(' || c === '[' || c === '{') {
            stack.push(c)
        } else if (
            (c === ')' && stack.length > 0 && stack[stack.length - 1] === '(') ||
            (c === '}' && stack.length > 0 && stack[stack.length - 1] === '{') ||
            (c === ']' && stack.length > 0 && stack[stack.length - 1] === '[')
        ) {
            stack.pop()
        } else {
            return false
        }
    }
    return stack.length === 0
}

export { isValid }

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