Leetcode 65: Valid Number

 

1. Problem Statement (Simplified)

You’re given a string s.

You must determine whether s is a valid number, according to these rules:

Valid forms include e.g.:

Integers: "2", "0089", "-0", "+10"

Decimals: "-0.1", "+3.14", "4.", "-." (not valid), "-.9", etc.

With exponent (scientific notation): "2e10", "-90E3", "3e+7", "+6e-1", "53.5e93", "-123.456e789"

Examples of invalid numbers: "abc", "1a", "1e", "e3", "99e2.5", "--6", "-+3", "95a54e53", ".", "e"

Roughly, valid number:

[optional sign] + [integer or decimal]

[optional exponent] – if exponent exists: 'e' or 'E' + [optional sign] + [digits]

2. Parsing Strategy – Single Pass with Flags

We parse string s from left to right and track:

seenDigit – have we seen any digit before exponent?

seenDot – have we seen a decimal point?

seenExp – have we seen exponent e or E?

seenDigitAfterExp – have we seen any digit after exponent?

Rules while scanning:

For each character c:

Digit ('0'..'9'):

seenDigit = true

If we already saw exponent (seenExp == true), also set seenDigitAfterExp = true.

Dot ('.'):

Valid only if:

We haven’t seen a dot before (!seenDot)

We haven’t seen exponent before (!seenExp)

Otherwise → invalid.

Exponent ('e' or 'E'):

Valid only if:

We haven’t seen exponent before (!seenExp)

We have already seen at least one digit before exponent (seenDigit)

Then:

seenExp = true

seenDigitAfterExp = false (we must see digits after exponent).

Sign ('+' or '-'):

Valid only at:

Beginning of the string (i == 0)

Immediately after exponent (s[i-1] == 'e' || s[i-1] == 'E')

Otherwise → invalid.

Anything else → invalid.

After scanning all characters:

Must have:

seenDigit == true

AND if exponent exists, seenDigitAfterExp == true (so exponent part must have at least one digit).

If those are satisfied → true, else false.

Pseudo-code:

3. Edge Cases to Consider

Only sign: "+", "-" → invalid (no digits).

Only dot: "." → invalid.

Sign with dot but no digits: ".", "+.", "-." → invalid (no digits).

Dot with digits: "0.", ".1", "3.", "3.5" → valid.

Exponent:

"1e", "e3" → invalid.

"1e+", "1e-" → invalid (no digits after sign).

"1e10", "1E-10" → valid.

No spaces are allowed by the problem statement, so we do not need to trim or handle spaces.

4. Java code

5. C code

6. C++ code

7. Python code

8. JavaScript code