update bunch

array/plus_one.py

@@ -4,6 +4,7 @@
 # The digits are stored such that the most significant
 # digit is at the head of the list.
 
+
 def plusOne(digits):
     """
     :type digits: List[int]

array/rotate_array.py

@@ -0,0 +1,30 @@
+"""
+Rotate an array of n elements to the right by k steps.
+
+For example, with n = 7 and k = 3,
+the array [1,2,3,4,5,6,7] is rotated to [5,6,7,1,2,3,4].
+
+Note:
+Try to come up as many solutions as you can,
+there are at least 3 different ways to solve this problem.
+"""
+
+
+def rotate(nums, k):
+    """
+    :type nums: List[int]
+    :type k: int
+    :rtype: void Do not return anything, modify nums in-place instead.
+    """
+    n = len(nums)
+    k = k % n
+    reverse(nums, 0, n - k - 1)
+    reverse(nums, n - k, n - 1)
+    reverse(nums, 0, n - 1)
+
+
+def reverse(array, a, b):
+    while a < b:
+        array[a], array[b] = array[b], array[a]
+        a += 1
+        b -= 1

array/three_sum.py

@@ -0,0 +1,46 @@
+"""
+Given an array S of n integers, are there elements a, b, c in S
+such that a + b + c = 0?
+Find all unique triplets in the array which gives the sum of zero.
+
+Note: The solution set must not contain duplicate triplets.
+
+For example, given array S = [-1, 0, 1, 2, -1, -4],
+
+A solution set is:
+[
+  [-1, 0, 1],
+  [-1, -1, 2]
+]
+"""
+
+
+def three_sum(nums:"List[int]")->"List[int]":
+    res = []
+    nums.sort()
+    for i in range(len(nums)-2):
+        if i > 0 and nums[i] == nums[i-1]:
+            continue
+        l, r = i+1, len(nums)-1
+        while l < r:
+            s = nums[i] + nums[l] + nums[r]
+            if s > 0:
+                r -= 1
+            elif s < 0:
+                l += 1
+            else:
+                # found three sum
+                res.append((nums[i], nums[l], nums[r]))
+                # remove duplicates
+                while l < r and nums[l] == nums[l+1]:
+                    l+=1
+                while l < r and nums[r] == nums[r-1]:
+                    r -= 1
+                l += 1
+                r -= 1
+    return res
+
+
+if __name__ == "__main__":
+    x = [-1,0,1,2,-1,-4]
+    print(three_sum(x))

array/two_sum.py

@@ -0,0 +1,29 @@
+"""
+Given an array of integers, return indices of the two numbers
+such that they add up to a specific target.
+
+You may assume that each input would have exactly one solution,
+and you may not use the same element twice.
+
+Example:
+    Given nums = [2, 7, 11, 15], target = 9,
+
+    Because nums[0] + nums[1] = 2 + 7 = 9,
+    return [0, 1].
+"""
+
+
+def two_sum(nums:"List[int]", target:"int")->"List[int]":
+    dic = {}
+    for i, num in enumerate(nums):
+        if num in dic:
+            return [dic[num], i]
+        else:
+            dic[target - num] = i
+
+
+if __name__ == "__main__":
+    arr = [3,2,4]
+    target = 6
+    res = two_sum(arr, target)
+    print(res)

backtrack/generate_parenthesis.py

@@ -0,0 +1,32 @@
+"""
+Given n pairs of parentheses, write a function to generate
+all combinations of well-formed parentheses.
+
+For example, given n = 3, a solution set is:
+
+[
+  "((()))",
+  "(()())",
+  "(())()",
+  "()(())",
+  "()()()"
+]
+"""
+
+def gen_parenthesis(n:"int")->"List[str]":
+    res = []
+    add_pair(res, "", n, 0)
+    return res
+
+def add_pair(res, s, left, right):
+    if left == 0 and right == 0:
+        res.append(s)
+        return
+    if right > 0:
+        add_pair(res, s+")", left, right-1)
+    if left > 0:
+        add_pair(res, s+"(", left-1, right+1)
+
+
+if __name__=="__main__":
+    print(gen_parenthesis(3))

backtrack/letter_combination.py

@@ -0,0 +1,35 @@
+"""
+Given a digit string, return all possible letter
+combinations that the number could represent.
+
+Input:Digit string "23"
+Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
+"""
+
+
+def letter_combinations(digits:"str")->"List[str]":
+    if digits == "":
+        return []
+    kmaps = {
+        "2": "abc",
+        "3": "def",
+        "4": "ghi",
+        "5": "jkl",
+        "6": "mno",
+        "7": "pqrs",
+        "8": "tuv",
+        "9": "wxyz"
+    }
+    ans = [""]
+    for num in digits:
+        tmp = []
+        for an in ans:
+            for char in kmaps[num]:
+                tmp.append(an + char)
+        ans = tmp
+    return ans
+
+
+if __name__ == "__main__":
+    digit_string = "23"
+    print(letter_combinations(digit_string))

backtrack/subsets.py

@@ -17,6 +17,9 @@
 # ]
 
 
+# O(2**n)
+
+
 def subsets(nums):
     res = []
     backtrack(res, nums, [], 0)
@@ -44,6 +47,13 @@ def backtrack(res, nums, stack, pos):
         # backtrack(res, nums, cur, pos+1)
 
 
+# Iteratively
+def subsets2(self, nums):
+    res = [[]]
+    for num in sorted(nums):
+        res += [item+[num] for item in res]
+    return res
+
 test = [1,2,3]
 print(test)
 print(subsets(test))

bit/count_ones.py

@@ -0,0 +1,21 @@
+"""
+Write a function that takes an unsigned integer and
+returns the number of ’1' bits it has
+(also known as the Hamming weight).
+
+For example, the 32-bit integer ’11' has binary
+representation 00000000000000000000000000001011,
+so the function should return 3.
+"""
+
+
+def count_ones(n):
+    """
+    :type n: int
+    :rtype: int
+    """
+    counter = 0
+    while n:
+        counter += n & 1
+        n >>= 1
+    return counter

bit/reverse_bits.py

@@ -0,0 +1,18 @@
+"""
+Reverse bits of a given 32 bits unsigned integer.
+
+For example, given input 43261596
+(represented in binary as 00000010100101000001111010011100),
+return 964176192
+(represented in binary as 00111001011110000010100101000000).
+"""
+
+
+def reverse_bits(n):
+    m = 0
+    i = 0
+    while i < 32:
+        m = (m << 1) + (n & 1)
+        n >>= 1
+        i += 1
+    return m

bit/single_number.py

@@ -0,0 +1,19 @@
+"""
+Given an array of integers, every element appears
+twice except for one. Find that single one.
+
+Note:
+Your algorithm should have a linear runtime complexity.
+Could you implement it without using extra memory?
+"""
+
+
+def single_number(nums):
+    """
+    :type nums: List[int]
+    :rtype: int
+    """
+    i = 0
+    for num in nums:
+        i ^= num
+    return i

bit/single_number2.py

@@ -0,0 +1,46 @@
+"""
+Given an array of integers, every element appears
+three times except for one, which appears exactly once.
+Find that single one.
+
+Note:
+Your algorithm should have a linear runtime complexity.
+Could you implement it without using extra memory?
+"""
+
+
+"""
+32 bits for each integer.
+Consider 1 bit in it, the sum of each integer's corresponding bit
+(except for the single number)
+should be 0 if mod by 3. Hence, we sum the bits of all
+integers and mod by 3,
+the remaining should be the exact bit of the single number.
+In this way, you get the 32 bits of the single number.
+"""
+
+
+def single_number(nums):
+    """
+    :type nums: List[int]
+    :rtype: int
+    """
+    res = 0
+    for i in range(0, 32):
+        count = 0
+        for num in nums:
+            if ((num >> i) & 1):
+                count += 1
+        res |= ((count % 3) << i)
+    if res >= 2**31:
+        res -= 2**32
+    return res
+
+
+# Another awesome answer
+def single_number2(nums):
+    ones, twos = 0, 0
+    for i in range(len(nums)):
+        ones = (ones ^ nums[i]) & ~twos
+        twos = (twos ^ nums[i]) & ~ones
+    return ones