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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


  • 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:

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

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  • 水果禮盒

    Won­der­ful site, thanks for sharing !!

  • 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©;
    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