add 008

Author: andavid

README.md

@@ -36,6 +36,7 @@
 | 387  | [First Unique Character in a String][387]   |
 | 242  | [Valid Anagram][242]                        |
 | 125  | [Valid Palindrome][125]                     |
+| 8    | [String to Integer (atoi)][008]             |
 
 
 **其他**
@@ -67,3 +68,4 @@
 [387]: https://github.com/andavid/leetcode-java/blob/master/note/387/README.md
 [242]: https://github.com/andavid/leetcode-java/blob/master/note/242/README.md
 [125]: https://github.com/andavid/leetcode-java/blob/master/note/125/README.md
+[008]: https://github.com/andavid/leetcode-java/blob/master/note/008/README.md

note/008/README.md

@@ -0,0 +1,81 @@
+# [String to Integer (atoi)][title]
+
+## Description
+
+Implement `atoi` to convert a string to an integer.
+
+**Hint:** Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.
+
+**Notes:** It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.
+
+ 
+
+**Requirements for atoi:**
+
+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. If the correct value is out of the range of representable values, INT_MAX (2147483647) or INT_MIN (-2147483648) is returned.
+
+
+
+## 思路
+
+1. 忽略前面的连续空格
+2. 处理第一个非空字符，如果是 + 或者 -，记录一个标志位
+3. 然后依次读取字符，如果是非数字字符或者数值超过 INT 最大值退出
+4. 结果乘以标志位，和 INT 最大值、INT 最小值进行比较
+
+## [完整代码][src]
+
+```java
+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 sign = str.charAt(i);
+    int flag = 1;
+    if (sign == '-' || sign == '+') {
+      flag = (sign == '-') ? -1 : 1;
+      i++;
+    }
+
+    long result = 0;
+    while (i < len) {
+      char c = str.charAt(i++);
+      if (c < '0' || c > '9' || result > Integer.MAX_VALUE) {
+        break;
+      }
+      result = result * 10 + c - '0';s
+    }
+
+    result *= flag;
+    if (result > Integer.MAX_VALUE) {
+      return Integer.MAX_VALUE;
+    } else if (result < Integer.MIN_VALUE) {
+      return Integer.MIN_VALUE;
+    }
+
+    return (int)result;
+  }
+}
+```
+
+[title]: https://leetcode.com/problems/string-to-integer-atoi
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_008/Solution.java