Valid or Well Formed Parentheses | Part – 1

Objective: You have been asked to Write an algorithm to find Whether Given the Sequence of parentheses are well formed. This question was asked in the Amazon Interview.

Input: A String contains a sequence of parentheses

Output: true or false on whether parentheses are well formed or not

Approach:

  • Idea is to have two counters, one for open parentheses ‘{‘ and one for close ‘}’
  • Read one character at a time and increment one of the counters
  • If any given point of time count of close parentheses is greater than the open one, return false
  • If at the end both counters are equal, return true

Example: { { } { } } – Well formed

{ { } { = Not well formed

Complete Code:

public class WellFormedParentheses {
public Boolean isWellFormed(String strParentheses) {
if (strParentheses == null) {
return false;
}
// Idea is to have two counters, one for open parentheses '{' and one
// for close '}'
// Read one character at a time and increment one of the counters
// If any given point of time count of close parentheses is greater than
// the open one, return false
// If at the end both counters are equal, return true
int openParenCounter = 0;
int closeParenCounter = 0;
for (int i = 0; i < strParentheses.length(); i++) {
char x = strParentheses.charAt(i);
if (x == '{')
openParenCounter++;
else if (x == '}')
closeParenCounter++;
if (closeParenCounter > openParenCounter) {
return false;
}
}
if (openParenCounter == closeParenCounter)
return true;
else
return false;
}
public static void main(String args[]) {
String x1 = "{{{{}}}}{}{}{}{}{}{}{}{}{}{}{{{}}}";
String x2 = "{{{{}}}}{}{}{}{{}{}{}{}{}{}{}{{{}}}";
String x3 = "{}{";
String x4 = "}{";
String x5 = "{{{{{{{{}}}}}}}}";
WellFormedParentheses w = new WellFormedParentheses();
System.out.println("Is " + x1 + " well formed ??? :"
+ w.isWellFormed(x1));
System.out.println("Is " + x2 + " well formed ??? :"
+ w.isWellFormed(x2));
System.out.println("Is " + x3 + " well formed ??? :"
+ w.isWellFormed(x3));
System.out.println("Is " + x4 + " well formed ??? :"
+ w.isWellFormed(x4));
System.out.println("Is " + x5 + " well formed ??? :"
+ w.isWellFormed(x5));
}
}

Output:
Is {{{{}}}}{}{}{}{}{}{}{}{}{}{}{{{}}} well formed ??? :true
Is {{{{}}}}{}{}{}{{}{}{}{}{}{}{}{{{}}} well formed ??? :false
Is {}{ well formed ??? :false
Is }{ well formed ??? :false
Is {{{{{{{{}}}}}}}} well formed ??? :true

3 thoughts on “Valid or Well Formed Parentheses | Part – 1”

  1. My Short and concise solution
    public boolean isWellFormed(String strParentheses) {
    if (strParentheses == null || strParentheses.length() == 0) {
    return false;
    }
    Stack stack = new Stack();
    for (int i=0; i<strParentheses.length();i++) {
    Character c = strParentheses.charAt(i);
    if (c == '}' && (stack.isEmpty() || stack.pop() != '{')) return false;
    if (c == '{') stack.push(c);
    }
    return stack.isEmpty();
    }

    Reply
  2. given solution is wrong as for the test case “}{ well formed ??? :false” it will return true. so we have to use an stack for this probliem

    Reply

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