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94. Valid Number

hardAsked at Salesforce

Determine if a string represents a valid number. Salesforce uses this to test edge-case enumeration and finite-state-machine reasoning.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Source citations

Public interview reports confirming this problem appears in Salesforce loops.

  • Glassdoor (2026-Q1)— Salesforce uses similar validation for numeric field inputs in custom Apex.
  • LeetCode Discuss (2025)— Hard for the edge-case enumeration.

Problem

Given a string s, return whether s is a valid number. A valid number can be split into these components: a decimal number (followed optionally by 'e' or 'E' and an integer), or an integer (followed optionally by 'e' or 'E' and an integer). A decimal/integer can be signed. See LeetCode for the full grammar.

Constraints

  • 1 <= s.length <= 20
  • s consists of only English letters, digits (0-9), plus '+', minus '-', or dot '.'.

Examples

Example 1

Input
s = "0"
Output
true

Example 2

Input
s = "e"
Output
false

Example 3

Input
s = "."
Output
false

Example 4

Input
s = "2e10"
Output
true

Approaches

1. Regex

Match against a carefully crafted regex.

Time
O(n)
Space
O(n)
function isNumber(s) {
  return /^[+-]?(\d+\.?\d*|\.\d+)([eE][+-]?\d+)?$/.test(s);
}

Tradeoff: Works but Salesforce may forbid regex — they want to see FSM reasoning.

2. Finite State Machine

Track states: start, sign, integer, dot, fraction, exp, expSign, expInt. Transitions on each character.

Time
O(n)
Space
O(1)
function isNumber(s) {
  let seenDigit = false, seenDot = false, seenE = false, seenDigitAfterE = true;
  for (let i = 0; i < s.length; i++) {
    const c = s[i];
    if (c >= '0' && c <= '9') { seenDigit = true; seenDigitAfterE = true; }
    else if (c === '+' || c === '-') { if (i > 0 && s[i-1] !== 'e' && s[i-1] !== 'E') return false; }
    else if (c === '.') { if (seenDot || seenE) return false; seenDot = true; }
    else if (c === 'e' || c === 'E') { if (seenE || !seenDigit) return false; seenE = true; seenDigitAfterE = false; }
    else return false;
  }
  return seenDigit && seenDigitAfterE;
}

Tradeoff: Explicit FSM. Each transition is enforced via boolean flags. Salesforce's preferred answer.

Salesforce-specific tips

Salesforce uses similar validation for numeric Apex field inputs (Currency, Number with decimal places). They grade on whether you can enumerate all the edge cases (leading +/-, '.', 'e', '+/-' after e, trailing digits after e). Bonus signal: mention this is essentially a regular language — the regex is correct precisely because no infinite recursion is needed.

Common mistakes

  • Allowing '+' or '-' anywhere (not just start or after e/E).
  • Allowing '.' after 'e' — exponent must be integer.
  • Allowing 'e' without digits before it — '.e1' is invalid.

Follow-up questions

An interviewer at Salesforce may pivot to one of these next:

  • Parse and convert to number (LC 8, atoi).
  • Validate scientific notation only.
  • Support hex and octal.

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Output

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FAQ

Is regex acceptable?

Depends on the interviewer. Some forbid regex to test FSM reasoning; others accept it. Always offer both if you have time.

What about '.5' and '5.'?

Both are valid — at least one digit (before OR after the dot) is required. The FSM tracks 'seenDigit' to ensure this.

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