65. Valid Number

Problem:

Validate if a given string is numeric.

Some examples:
"0" => true
" 0.1 " => true
"abc" => false
"1 a" => false
"2e10" => true

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one.

Update (2015-02-10): The signature of the C++ function had been updated. If you still see your function signature accepts a const char * argument, please click the reload button to reset your code definition.

Solution:

JavaScript specific solutions:

ONE

• Math.abs will first convert the argument to number.
• Math.abs(' ') === 0.

/**
 * @param {string} s
 * @return {boolean}
 */
let isNumber = function (s) {
  return !!s.trim() && Math.abs(s) >= 0;
};

TWO

• isNaN will first convert the argument to number.
• isNaN(' ') === false.

/**
 * @param {string} s
 * @return {boolean}
 */
let isNumber = function (s) {
  return !!s.trim() && !isNaN(s);
};

THREE

General solution. Take a look at the ECMA Spec.

Similary, we can define our own syntax, which requires a few changes:

SignedDecimalLiteral::
  DecimalLiteral
  + DecimalLiteral
  - DecimalLiteral

DecimalLiteral::
  DecimalDigits . [DecimalDigits] [ExponentPart]
  . DecimalDigits [ExponentPart]
  DecimalDigits [ExponentPart]

DecimalDigits::
  DecimalDigit
  DecimalDigits DecimalDigit

DecimalDigit:: one of
  0123456789

ExponentPart::
  ExponentIndicator SignedInteger

ExponentIndicator::one of
  eE

SignedInteger::
  DecimalDigits
  + DecimalDigits
  - DecimalDigits

Now implement the parser. It is much easier now because we have a clear mental map of the syntax.

/**
 * @param {string} s
 * @return {boolean}
 */
let isNumber = function (s) {
  let start = 0;
  while (s[start] === " ") {
    start++;
  }
  if (s[start] === "+" || s[start] === "-") {
    start++;
  }
  let nextIndex = parseDecimalLiteral(s, start);
  while (s[nextIndex] === " ") {
    nextIndex++;
  }
  return nextIndex === s.length;
};

/**
 * @param {string} s
 * @param {number} start - start index
 * @return {number} next index, -1 means error
 */
function parseDecimalLiteral(s, start) {
  let nextIndex = -1;
  if (s[start] === ".") {
    nextIndex = parseDecimalDigits(s, start + 1);
    if (nextIndex === -1) {
      return -1;
    }
  } else {
    nextIndex = parseDecimalDigits(s, start);
    if (nextIndex === -1) {
      return -1;
    }

    if (s[nextIndex] === ".") {
      const optNextIndex = parseDecimalDigits(s, ++nextIndex);
      if (optNextIndex !== -1) {
        nextIndex = optNextIndex;
      }
    }
  }

  const optNextIndex = parseExponentPart(s, nextIndex);
  return optNextIndex === -1 ? nextIndex : optNextIndex;
}

/**
 * @param {string} s
 * @param {number} start - start index
 * @return {number} next index, -1 means error
 */
function parseDecimalDigits(s, start) {
  if (start === s.length) {
    return -1;
  }

  for (let i = start; i < s.length; i++) {
    const digit = s.charCodeAt(i) - 48;
    if (!(digit >= 0 && digit <= 9)) {
      return i === start ? -1 : i;
    }
  }
  return s.length;
}

/**
 * @param {string} s
 * @param {number} start - start index
 * @return {number} next index, -1 means error
 */
function parseDecimalIntegerLiteral(s, start) {
  if (start === s.length) {
    return -1;
  }

  let nextIndex = start;
  if (s[start] === "0") {
    nextIndex++;
  }

  const digit = s.charCodeAt(nextIndex) - 48;
  if (!(digit > 0 && digit <= 9)) {
    return nextIndex === start ? -1 : nextIndex;
  }
  nextIndex++;

  const optNextIndex = parseDecimalDigits(s, nextIndex);
  return optNextIndex === -1 ? nextIndex : optNextIndex;
}

/**
 * @param {string} s
 * @param {number} start - start index
 * @return {number} next index, -1 means error
 */
function parseExponentPart(s, start) {
  if (s[start] !== "e" && s[start] !== "E") {
    return -1;
  }

  let nextIndex = start + 1;
  if (s[nextIndex] === "+" || s[nextIndex] === "-") {
    nextIndex++;
  }

  return parseDecimalDigits(s, nextIndex);
}

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