proper nesting

properNesting.js

// Carlos Martinez @coxato

// the problem is about validate and fill a string with square brackets, curly brackets and braces () [] {}
// if you have for example
// "]-(" you have to return "[]-()"
// "[{{" returns "[{{}}]"
// "" returns ""
// "} { [()] (()v()) } ])" returns "([{} { [()] (()v()) } ])"
// "(]" returns null
// "[}}[" returns null


const openChar = {
    '{': '}',
    '[': ']',
    '(': ')'
}

const closeChar = {
    '}': '{',
    ']': '[',
    ')': '('
}

// save here the completed string, if we need to complete of course
let completedString;


function validateAndComplete(str) {
    // stack for save the {[( characters
    let openStack = [];
    
    for (let i = 0; i < str.length; i++) {
        let char = str[i];    

        if(openChar[char]){
            openStack.push(char);
        }
        else if(closeChar[char]){
            const lastOpenChar = openStack.pop();

            if(lastOpenChar){
                if(openChar[lastOpenChar] !== char) return false;
            }
            // if it's undefined, check if we can complete the string
            // this also means that we have a ), ] or } that can be added at the beginning of the string
            else {

                completedString = closeChar[char] + str;
                return validateAndComplete(completedString);
            }
        }
    }

    // if some (, [ or { is still inside the stack means
    // that we can take it and add the corresponding ), ] or } at end of string
    if(openStack.length){
        completedString = str + openChar[openStack.pop()];
        return validateAndComplete(completedString);
    }

    return true;
}


function nest(str) {
    completedString = ''; // reset

    if(validateAndComplete(str)){
        return completedString || str;
    };

    return null;
}

// some test cases
// const testCases = [
//     "( )[ ]",
//     "]-(",
//     "}(", // I dunno why should be null, I think {}()
//     "([{}{}]())",
//     "",
//     "([hi]) { hello [] (nice) {123} }",
//     "[}}[",
//     ")()] {[]} (",
//     "[{{",
//     "{ [] (}",
//     "(]",
//     "} { [()] (()v()) } ])"
// ];

// testCases.forEach( t => {
//     console.log(t, '->', nest(t));
// })