[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:

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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).
Location: Philadelphia, PA, USA

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