Add explanations for problem 3, 8, 151.

Author: qiyuangong

README.md

@@ -4,10 +4,13 @@
 
 | # | Title | Solution | Basic idea (One line) |
 |---| ----- | -------- | ---------- |
-| 1 | [Two Sum](https://leetcode.com/problems/two-sum/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/1_Two_Sum.py) | 1. hash 2. Sort and search with two points|
-| 15 | [3Sum](https://leetcode.com/problems/3sum/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/15_3Sum.py) | 1. Sort and O(n^2) search with three points 2. Multiple Two Sum (Problem 1) |
+| 1 | [Two Sum](https://leetcode.com/problems/two-sum/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/1_Two_Sum.py) | 1. hash <br>2. Sort and search with two points |
+| 3 | [Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/3_Longest_Substring_Without_Repeating_Characters.py) |1. Check every possible substring O(n^2) <br>2. Remember the character index and current check pos, if character index >= current pos, then there is duplicate |
+| 8 | [String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/8_String_to_Integer(atoi).py) | Overflow, Space, and negative number |
+| 15 | [3Sum](https://leetcode.com/problems/3sum/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/15_3Sum.py) | 1. Sort and O(n^2) search with three points <br>2. Multiple Two Sum (Problem 1) |
 | 16 | [3Sum Closest](https://leetcode.com/problems/3sum-closest/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/16_3Sum_Closest.py) | Sort and Multiple Two Sum check abs|
 | 18 | [4Sum](https://leetcode.com/problems/4sum/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/18_4Sum.py) | The same as 3Sum, but we can merge pairs with the same sum |
-| 28 | [Implement strStr()](https://leetcode.com/problems/implement-strstr/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/28_Implement_strStr().py) | 1. O(nm) comparison 2. KMP|
+| 28 | [Implement strStr()](https://leetcode.com/problems/implement-strstr/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/28_Implement_strStr().py) | 1. O(nm) comparison <br>2. KMP |
+| 65 | [Valid Number](https://leetcode.com/problems/valid-number/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/65_Valid_Number.py) | 1. strip leading and tailing space, then check float using exception, check e using split <br>2. check is number split by . or e, note that number after e may be negative |
 | 125 | [Valid Palindrome](https://leetcode.com/problems/valid-palindrome/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/125_Valid_Palindrome.py)| Exclude non-alphanumeric characters and compare O(n) |
-||
+| 151 | [Reverse Words in a String](https://discuss.leetcode.com/category/159/reverse-words-in-a-string) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/151_Reverse_Words_in_a_String.py)| 1. Python split by space <br>2. Reverse all and reverse words|

python/151_Reverse_Words_in_a_String.py

@@ -1,12 +1,39 @@
 class Solution(object):
+    # def reverseWords(self, s):
+    #     """
+    #     :type s: str
+    #     :rtype: str
+    #     """
+    #     # O(n) and O(n) space
+    #     if len(s) == 0:
+    #         return s
+    #     temp = s.split(' ')
+    #     temp = [t for t in temp if len(t) > 0]
+    #     temp = reversed(temp)
+    #     return ' '.join(temp)
+
     def reverseWords(self, s):
-        """
-        :type s: str
-        :rtype: str
-        """
-        if len(s) == 0:
-            return s
-        temp = s.split(' ')
-        temp = [t for t in temp if len(t) > 0]
-        temp = reversed(temp)
-        return ' '.join(temp)
+        # remove tail space
+        s = s.strip(' ')
+        array_s = []
+        last = ' '
+        # remove multiple spaces
+        for i in range(len(s)):
+            if s[i] != ' ':
+                array_s.append(s[i])
+            else:
+                if last != ' ':
+                    array_s.append(s[i])
+            last = s[i]
+        array_s = array_s[::-1]
+        ls, pos = len(array_s), 0
+        for i in range(ls + 1):
+            if i == ls or array_s[i] == ' ':
+                self.reverse(array_s, pos, i)
+                pos = i + 1
+        return ''.join(array_s)
+
+    def reverse(self, array_s, begin, end):
+        for i in range((end - begin) / 2):
+            array_s[begin + i], array_s[end - i - 1] = array_s[end - i - 1], array_s[begin + i]
+

python/220_Contains_Duplicate_III.py

@@ -0,0 +1,8 @@
+class Solution(object):
+    def containsNearbyAlmostDuplicate(self, nums, k, t):
+        """
+        :type nums: List[int]
+        :type k: int
+        :type t: int
+        :rtype: bool
+        """

python/3_Longest_Substring_Without_Repeating_Characters.py

@@ -62,6 +62,8 @@ def lengthOfLongestSubstring(self, s):
         ls = len(s)
         i = max_len = 0
         for j in range(ls):
+            # Note that when charMap[ord(s[j])] >= i, it means that there are
+            # duplicate character in current i,j. So we need to update i.
             if charMap[ord(s[j])] >= i:
                 i = charMap[ord(s[j])] + 1
             charMap[ord(s[j])]  = j

python/65_Valid_Number.py

@@ -1,24 +1,52 @@
 class Solution(object):
+    # def isNumber(self, s):
+    #     """
+    #     :type s: str
+    #     :rtype: bool
+    #     """
+    #     # remove lead and tail space
+    #     s = s.strip()
+    #     try:
+    #         float(s)
+    #         return True
+    #     except:
+    #         if '.' in s or ' ' in s:
+    #             return False
+    #         temp = s.split('e')
+    #         if len(temp) == 2:
+    #             try:
+    #                 int(temp[0])
+    #                 int(temp[1])
+    #             except:
+    #                 return False
+    #             return True
+    #     return False
+
     def isNumber(self, s):
-        """
-        :type s: str
-        :rtype: bool
-        """
-        # remove lead and tail space
-        s.strip()
-        try:
-            float(s)
+        s = s.strip()
+        ls, pos = len(s), 0
+        if ls == 0:
+            return False
+        if s[pos] == '+' or s[pos] == '-':
+            pos += 1
+        isNumeric = False
+        while pos < ls and s[pos].isdigit():
+            pos += 1
+            isNumeric = True
+        if pos < ls and s[pos] == '.':
+            pos += 1
+            while pos < ls and s[pos].isdigit():
+                pos += 1
+                isNumeric = True
+        elif pos < ls and s[pos] == 'e' and isNumeric:
+            isNumeric = False
+            pos += 1
+            if pos < ls and (s[pos] == '+' or s[pos] == '-'):
+                pos += 1
+            while pos < ls and s[pos].isdigit():
+                pos += 1
+                isNumeric = True
+        print pos, ls, isNumeric
+        if pos == ls and isNumeric:
             return True
-        except:
-            if '.' in s or ' ' in s:
-                return False
-            temp = s.split('e')
-            if len(temp) == 2:
-                try:
-                    int(temp[0])
-                    int(temp[1])
-                except:
-                    return False
-                return True
         return False
-