Merge pull request #24 from alberthu16/day35

Day 35: reverse string pt 2

Author: Albert Hu

day35/README.md

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+Question of the day: https://leetcode.com/problems/reverse-string-ii/#/description
+
+Given a string and an integer k, you need to reverse the first k characters
+for every 2k characters counting from the start of the string. If there are
+less than k characters left, reverse all of them. If there are less than
+2k but greater than or equal to k characters, then reverse the first k
+characters and leave the rest as they are.
+
+Example:
+
+```
+Input: s = "abcdefg", k = 2  
+Output: "bacdfeg"  
+```
+
+Restrictions:
+
+1. The string consists of lowercase English letters only.  
+2. Length of the given string and k will in the range [1, 10000]
+
+## Ideas
+
+1. break the sting into chunks of length `2k`  
+2. reverse up to the first `k` of each chunk  
+3. join the chunks together and return the result  
+
+
+Although I logically want to break the string up into `2k` chunks right away,
+I can just use some pointers as I move down the string so that I don't allocate
+too much unnecessary memory. Reversing the relevant indices and returning at the
+end will take `O(n)` runtime and `O(1)` space.
+
+## Code
+[Python](./reverseStr.py)
+
+## Follow-up
+
+Try writing this in C.

day35/reverseStr.py

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+def reverseStr(s, k):
+    # edge
+    size = len(s)
+    if size < k:
+        return s[::-1]
+
+    # convert string to list
+    letters = list(s)
+    i = 0
+
+    # reverse all the full chunks
+    while i + 2*k-1 < size:
+        reverse(letters, i, i+k-1)
+        i += 2*k
+
+    # take care of the remainder
+    if i+k-1 < size:
+        reverse(letters, i, i+k-1)
+    else:
+        reverse(letters, i, size-1)
+
+    # fin
+    return "".join(letters)
+
+# reverses the indices of arr between start and end, in place
+def reverse(arr, start, end):
+    while start < end:
+        arr[start], arr[end] = arr[end], arr[start]
+        start += 1
+        end -= 1
+
+def testReverse():
+    t1 = [0, 1, 2, 3, 4]
+    reverse(t1, 0, 2)
+    assert t1 == [2, 1, 0, 3, 4]
+
+    t2 = [0, 1, 2, 3, 4]
+    reverse(t2, 1, 3)
+    assert t2 == [0, 3, 2, 1, 4]
+
+def testReverseStr():
+    assert reverseStr("", 1) == ""
+    assert reverseStr("", 2) == ""
+    assert reverseStr("", 10000) == ""
+    assert reverseStr("abcdefg", 1) == "abcdefg"
+    assert reverseStr("abcdefg", 2) == "bacdfeg"
+    assert reverseStr("abcdefg", 3) == "cbadefg"
+    assert reverseStr("abcdefg", 4) == "dcbaefg"
+    assert reverseStr("abcdefg", 5) == "edcbafg"
+    assert reverseStr("abcdefg", 6) == "fedcbag"
+    assert reverseStr("abcdefg", 7) == "gfedcba"
+    assert reverseStr("abcdefg", 8) == "gfedcba"
+    assert reverseStr("abcdefg", 9) == "gfedcba"
+
+def main():
+    testReverse()
+    testReverseStr()
+
+if __name__ == "__main__":
+    main()