8.string-to-integer-atoi.java

/*
 * @lc app=leetcode id=8 lang=java
 *
 * [8] String to Integer (atoi)
 *
 * https://leetcode.com/problems/string-to-integer-atoi/description/
 *
 * algorithms
 * Medium (14.50%)
 * Total Accepted:    346.6K
 * Total Submissions: 2.4M
 * Testcase Example:  '"42"'
 *
 * Implement atoi which converts a string to an integer.
 *
 * The function first discards as many whitespace characters as necessary until
 * the first non-whitespace character is found. Then, starting from this
 * character, takes an optional initial plus or minus sign followed by as many
 * numerical digits as possible, and interprets them as a numerical value.
 *
 * The string can contain additional characters after those that form the
 * integral number, which are ignored and have no effect on the behavior of
 * this function.
 *
 * If the first sequence of non-whitespace characters in str is not a valid
 * integral number, or if no such sequence exists because either str is empty
 * or it contains only whitespace characters, no conversion is performed.
 *
 * If no valid conversion could be performed, a zero value is returned.
 *
 * Note:
 *
 *
 * Only the space character ' ' is considered as whitespace character.
 * Assume we are dealing with an environment which could only store integers
 * within the 32-bit signed integer range: [−2^31,  2^31 − 1]. If the numerical
 * value is out of the range of representable values, INT_MAX (2^31 − 1) or
 * INT_MIN (−2^31) is returned.
 *
 *
 * Example 1:
 *
 *
 * Input: "42"
 * Output: 42
 *
 *
 * Example 2:
 *
 *
 * Input: "   -42"
 * Output: -42
 * Explanation: The first non-whitespace character is '-', which is the minus
 * sign.
 * Then take as many numerical digits as possible, which gets 42.
 *
 *
 * Example 3:
 *
 *
 * Input: "4193 with words"
 * Output: 4193
 * Explanation: Conversion stops at digit '3' as the next character is not a
 * numerical digit.
 *
 *
 * Example 4:
 *
 *
 * Input: "words and 987"
 * Output: 0
 * Explanation: The first non-whitespace character is 'w', which is not a
 * numerical
 * digit or a +/- sign. Therefore no valid conversion could be performed.
 *
 * Example 5:
 *
 *
 * Input: "-91283472332"
 * Output: -2147483648
 * Explanation: The number "-91283472332" is out of the range of a 32-bit
 * signed integer.
 * Thefore INT_MIN (−2^31) is returned.
 *
 */
class Solution {
  public int myAtoi(String str) {
    if (str == null || str.length() == 0) {
      return 0;
    }

    int len = str.length();
    int i = 0;
    while (i < len && str.charAt(i) == ' ') {
      i++;
    }
    if (i >= len) {
      return 0;
    }

    char ch = str.charAt(i);
    int flag = 1;
    if (ch == '+') {
      i++;
    } else if (ch == '-') {
      flag = -1;
      i++;
    }

    long result = 0;
    while (i < len) {
      ch = str.charAt(i++);
      if (ch < '0' || ch > '9' || result > Integer.MAX_VALUE) {
        break;
      }
      result = result * 10 + (ch - '0');
    }

    result *= flag;

    if (result > Integer.MAX_VALUE) {
      return Integer.MAX_VALUE;
    }
    if (result < Integer.MIN_VALUE) {
      return Integer.MIN_VALUE;
    }

    return (int)result;
  }
}