[LeetCode] 65. Valid Number

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Problem

Validate if a given string can be interpreted as a decimal number.

Some examples:

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"0" => true
" 0.1 " => true
"abc" => false
"1 a" => false
"2e10" => true
" -90e3   " => true
" 1e" => false
"e3" => false
" 6e-1" => true
" 99e2.5 " => false
"53.5e93" => true
" --6 " => false
"-+3" => false
"95a54e53" => false

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one. However, here is a list of characters that can be in a valid decimal number:

  • Numbers 0-9
  • Exponent - “e”
  • Positive/negative sign - “+”/”-“
  • Decimal point - “.”

Of course, the context of these characters also matters in the input.

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.

Explanation

  1. Loop through all characters of the string. Initialize variables boolean num = false, numAfterE = true, dot = false, exp = false, sign = false to mark if we have seen these variable characteristic, then check different conditions.

Solution

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class Solution {
    public boolean isNumber(String s) {
        boolean num = false, numAfterE = true, dot = false, exp = false, sign = false;
        int n = s.length();
        for (int i = 0; i < n; i++) {
            char c = s.charAt(i);
            if (c == ' ') {
                if (i < n-1 && s.charAt(i+1) != ' ' && (num || dot || exp || sign)) {
                    return false;
                }
            }
            else if (c == '+' || c == '-') {
                if (i > 0 && s.charAt(i-1) != 'e' && s.charAt(i-1) != ' ') {
                    return false;
                }
                sign = true;
            }
            else if (c >= '0' && c <= '9') {
                num = true;
                numAfterE = true;
            }
            else if (c == '.') {
                if (dot || exp) {
                    return false;
                }
                dot = true;
            }
            else if (c == 'e') {
                if (exp || !num) {
                    return false;
                }
                exp = true;
                numAfterE = false;
            }
            else {
                return false;
            }
        }
        
        return num && numAfterE;
    }
}