AI solution for JavaScript Coding Question on BFE.dev
173. uncompress string

Here's a possible solution in JavaScript:

function uncompress(str) {
  let stack = [];
  let currentStr = '';
  let currentNum = 0;

  for (let i = 0; i < str.length; i++) {
    let c = str.charAt(i);

    if (isDigit(c)) {
      currentNum = currentNum * 10 + parseInt(c);
    } else if (c === '(') {
      stack.push(currentStr);
      stack.push(currentNum);
      currentStr = '';
      currentNum = 0;
    } else if (c === ')') {
      let k = stack.pop();
      let prevStr = stack.pop();
      currentStr = prevStr + currentStr.repeat(k);
    } else {
      currentStr += c;
    }
  }

  return currentStr;
}

function isDigit(c) {
  return c >= '0' && c <= '9';
}

The idea is to use a stack to keep track of the strings and numbers that we encounter while iterating through the input string. We maintain two variables currentStr and currentNum, which represent the substring we're currently building and the number of times we should repeat it, respectively. Whenever we encounter a digit, we add it to currentNum. Whenever we encounter an opening parenthesis, we push the current state onto the stack and start a new one. Whenever we encounter a closing parenthesis, we pop the previous state from the stack and use it to repeat the current substring currentStr k times. Finally, whenever we encounter a non-special character, we simply add it to currentStr.

Overall, this algorithm runs in linear time O(n), where n is the length of the input string, since we only iterate once through the input and perform constant-time operations with the stack and string manipulation.