# Find Whether Given Sequence of parentheses are well formed.

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:

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

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If you find anything incorrect or you feel that there is any better approach to solve the above problem, please write comment.
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• Chandra Gopalaiah

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();
}

• Rahul Batheja

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