solves 8: string to atoi

Author: anishLearnsToCode

Diff for: README.md

@@ -17,6 +17,7 @@
 |  5   | [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring)                                                              |                                                           [![Java](assets/java.png)](src/LongestPalindromicSubstring.java)                                                           |                                                                                                                                   |
 |  6   | [Zigzag Conversion](https://leetcode.com/problems/zigzag-conversion)                                                                                      |                                                                [![Java](assets/java.png)](src/ZigZagConversion.java)                                                                 |                                                                                                                                   |
 |  7   | [Reverse Integer](https://leetcode.com/problems/reverse-integer/)                                                                                         |                                    [![Java](assets/java.png)](src/ReverseInteger.java) [![Python](assets/python.png)](python/reverse_integer.py)                                     | [![java-yt](assets/java-yt.png)](https://youtu.be/7bOhyl5lWjI) [![python-yt](assets/python-yt.png)](https://youtu.be/lmLG30TLcSg) |
+|  8   | [String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi)                                                                          |                                                               [![Java](assets/java.png)](src/StringToIntegerAtoi.java)                                                               |                                                                                                                                   |
 |  9   | [PalindromeNumber](https://leetcode.com/problems/palindrome-number/)                                                                                      |                                  [![Java](assets/java.png)](src/PalindromeNumber.java) [![Python](assets/python.png)](python/palindrome_number.py)                                   |                                                                                                                                   |
 |  13  | [Roman To Integer](https://leetcode.com/problems/roman-to-integer/)                                                                                       |                                    [![Java](assets/java.png)](src/RomanToInteger.java) [![Python](assets/python.png)](python/roman_to_integer.py)                                    | [![java-yt](assets/java-yt.png)](https://youtu.be/BCue_mO_81A) [![python-yt](assets/python-yt.png)](https://youtu.be/8h_yGTNvKMA) |
 |  14  | [Longest Common Prefix](https://leetcode.com/problems/longest-common-prefix/)                                                                             |                               [![Java](assets/java.png)](src/LongestCommonPrefix.java) [![Python](assets/python.png)](python/longest_common_prefix.py)                               |                                                                                                                                   |

Diff for: src/StringToIntegerAtoi.java

@@ -0,0 +1,45 @@
+// https://leetcode.com/problems/string-to-integer-atoi
+// T:O(|input|)
+// S:O(1)
+
+public class StringToIntegerAtoi {
+    public int myAtoi(String input) {
+        int sign = 1;
+        int result = 0;
+        int index = 0;
+        int n = input.length();
+
+        // Discard all spaces from the beginning of the input string.
+        while (index < n && input.charAt(index) == ' ') {
+            index++;
+        }
+
+        // sign = +1, if it's positive number, otherwise sign = -1.
+        if (index < n && input.charAt(index) == '+') {
+            index++;
+        } else if (index < n && input.charAt(index) == '-') {
+            sign = -1;
+            index++;
+        }
+
+        // Traverse next digits of input and stop if it is not a digit
+        while (index < n && Character.isDigit(input.charAt(index))) {
+            int digit = input.charAt(index) - '0';
+
+            // Check overflow and underflow conditions.
+            if ((result > Integer.MAX_VALUE / 10) ||
+                    (result == Integer.MAX_VALUE / 10 && digit > Integer.MAX_VALUE % 10)) {
+                // If integer overflowed return 2^31-1, otherwise if underflowed return -2^31.
+                return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
+            }
+
+            // Append current digit to the result.
+            result = 10 * result + digit;
+            index++;
+        }
+
+        // We have formed a valid number without any overflow/underflow.
+        // Return it after multiplying it with its sign.
+        return sign * result;
+    }
+}