用栈来解决这类问题, 很简单

/**
 * @param {string} S
 * @return {string}
 */
const removeDuplicates = function (S) {
  let stackString = '';
  for (let i = 0; i < S.length; i++) {
    const element = S[i];
    if (stackString.length === 0) {
      stackString += element;
    } else if (stackString.length !== 0 && stackString[stackString.length - 1] === element) {
      while (stackString[stackString.length - 1] === element) {
        stackString = stackString.slice(0, stackString.length - 1);
      }
    } else {
      stackString += element;
    }
  }
  return stackString;
};