8. String to Integer (atoi)

难度: Medium

题目描述

Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer.

The algorithm for myAtoi(string s) is as follows:

1. Whitespace: Ignore any leading whitespace (" ").
2. Signedness: Determine the sign by checking if the next character is '-' or '+', assuming positivity if neither present.
3. Conversion: Read the integer by skipping leading zeros until a non-digit character is encountered or the end of the string is reached. If no digits were read, then the result is 0.
4. Rounding: If the integer is out of the 32-bit signed integer range [-231, 231 - 1], then round the integer to remain in the range. Specifically, integers less than -231 should be rounded to -231, and integers greater than 231 - 1 should be rounded to 231 - 1.

Return the integer as the final result.

 

Example 1:

Input: s = "42"

Output: 42

Explanation:

The underlined characters are what is read in and the caret is the current reader position.
Step 1: "42" (no characters read because there is no leading whitespace)
         ^
Step 2: "42" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "42" ("42" is read in)
           ^

Example 2:

Input: s = " -042"

Output: -42

Explanation:

Step 1: "   -042" (leading whitespace is read and ignored)
            ^
Step 2: "   -042" ('-' is read, so the result should be negative)
             ^
Step 3: "   -042" ("042" is read in, leading zeros ignored in the result)
               ^

Example 3:

Input: s = "1337c0d3"

Output: 1337

Explanation:

Step 1: "1337c0d3" (no characters read because there is no leading whitespace)
         ^
Step 2: "1337c0d3" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "1337c0d3" ("1337" is read in; reading stops because the next character is a non-digit)
             ^

Example 4:

Input: s = "0-1"

Output: 0

Explanation:

Step 1: "0-1" (no characters read because there is no leading whitespace)
         ^
Step 2: "0-1" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "0-1" ("0" is read in; reading stops because the next character is a non-digit)
          ^

Example 5:

Input: s = "words and 987"

Output: 0

Explanation:

Reading stops at the first non-digit character 'w'.

 

Constraints:

• 0 <= s.length <= 200
• s consists of English letters (lower-case and upper-case), digits (0-9), ' ', '+', '-', and '.'.

解题思路

代码实现

解决方案

java

class Solution {
    public int myAtoi(String str) {
        
        if (null == str || "".equals(str.trim())) {
            return 0;
        }

        int start = -1;
        int end = -1;
        char first = ' ';
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);

            if (start == -1) {
                if (Character.isSpaceChar(c)) {
                    continue;
                } else if (Character.isDigit(c)) {
                    if (start == -1) {
                        start = i;
                    }
                    end = i;
                } else if (start == -1 && first == ' ' && ('-' == c || '+' == c)) {
                    first = c;
                    start = i;
                    end = i;

                } else {
                    break;
                }


            } else {

                if (Character.isDigit(c)) {

                    end = i;
                } else {
                    break;
                }

            }

        }

        boolean flagOne = first != ' ' && end - start >0 && start > -1 && end > -1;
        boolean flagTwo = first == ' ' && end - start >= 0 && start > -1 && end > -1;


        if (flagOne || flagTwo) {
            boolean flag = true;

            if (first == '-') {
                flag = false;
            }
            if (first != ' ') {
                start++;
            }

            String subString = str.substring(start, end + 1);
            long temp = 0;

            for (int i = 0; i < subString.length(); i++) {

                temp += Character.getNumericValue(subString.charAt(i)) * Math.pow(10, subString.length() - i - 1);

            }


            if (!flag) {
                temp = 0 - temp;
            }
            if (temp > Integer.MAX_VALUE) {
                return Integer.MAX_VALUE;
            }

            if (temp < Integer.MIN_VALUE) {
                return Integer.MIN_VALUE;
            }

            return (int) temp;

        }


        return 0;
    }
}