[LeetCode] 65. Valid Number

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.

Thought process:

There are several rules to enforce:

1. There must be at least one number.
2. e must come after some number and be followed by some number.
3. Point cannot come after e.
4. + and - can only appear in two positions: at the start of the string or right after e.
5. There cannot be characters other than digits, e, point, + or -, or space.

We can use several boolean flags to make these happen.

Solution:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

class Solution { public boolean isNumber(String s) { s = s.trim(); boolean eSeen = false; boolean numberSeen = false; boolean pointSeen = false; boolean numberAfterE = false; for (int i = 0; i < s.length(); i++) { char c = s.charAt(i); if (Character.isDigit(c)) { numberSeen = true; numberAfterE = true; } else if (c == 'e') { if (eSeen || !numberSeen) { return false; } eSeen = true; numberAfterE = false; } else if (c == '.') { if (eSeen || pointSeen) { return false; } pointSeen = true; } else if (c == '+' || c == '-') { if (i != 0 && s.charAt(i - 1) != 'e') { return false; } } else { return false; } } return numberSeen && numberAfterE; } }

Time complexity: O(n).