Bernard Wong
  • Software Engineer
  • Problems
    • Lowest Common Ancestor of a Binary Tree
    • Longest Substring Without Repeating Characters
    • Longest Palindromic Substring
    • Longest Common Prefix
    • Isomorphic Strings
    • Integer to Roman
    • Frog Jump
    • Find the Difference
    • Find k closest elements to a given value
    • Longest Common Subsequence
    • Binary Search Tree from Sorted Array
    • Balanced Binary Tree
    • Sort Using Two Stacks
    • O(1) Stack
    • k-th element to last of a LinkedList
    • Dedup LinkedList
    • Check Rotated String
    • Compress String by Character Count
    • Escape HTML whitespace
    • Check String Permutation
    • Unique String
    • Container With Most Water
    • 4 Sum
    • 3 Sum Closest
    • 3 Sum
    • 2 Sum
    • Maximum Subarray
    • Nested List Weight Sum
    • Palindrome Number
    • Pow(x, n)
    • Regular Expression Matching
    • Remove Nth Node From End of List
    • Reverse Integer
    • Roman to Integer
    • Rotate Array
    • Search a 2D Matrix
    • Shortest Word Distance
    • Two Sum III - Data structure design
    • Valid Parentheses
    • ZigZag Conversion
    • Quicksort
    • Add Two Numbers
    • Best Time to Buy and Sell Stock
    • Letter Combinations of a Phone Number
  • Data Structures
    • Heap
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  1. Problems

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.

val brackets:Map[Char, Char] = Map[Char, Char] (
  ')' -> '(',
  '}' -> '{',
  ']' -> '['
)

def isValid(s: String): Boolean = {
  var lst: List[Char] = Nil
  for (c <- s) {
    if (brackets contains c) {
      for (open <- brackets.get(c)) {
        if (lst.head == open) lst = lst.tail
        else return false
      }
    } else {
      lst = c :: lst
    }
  }
  lst.isEmpty
}

isValid("()")
isValid("()[]{}")
isValid("(]")
isValid("([)]")

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Last updated 6 years ago