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} */ var 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} */ var 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}
 */
var 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)
}

Template generated via Leetmark.