diff --git a/Week_02/.DS_Store b/Week_02/.DS_Store
new file mode 100644
index 00000000..6761ee2a
Binary files /dev/null and b/Week_02/.DS_Store differ
diff --git a/Week_02/id_55/LeetCode_236_55.py b/Week_02/id_55/LeetCode_236_55.py
new file mode 100644
index 00000000..9e26a314
--- /dev/null
+++ b/Week_02/id_55/LeetCode_236_55.py
@@ -0,0 +1,17 @@
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+class Solution:
+    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
+        if root in (None, p, q): 
+            return root
+        left=self.lowestCommonAncestor(root.left, p, q)
+        right=self.lowestCommonAncestor(root.right, p, q)
+        if left and right:
+            return root
+        else:
+            return left or right
diff --git a/Week_02/id_55/LeetCode_242_55.py b/Week_02/id_55/LeetCode_242_55.py
new file mode 100644
index 00000000..1f5851e4
--- /dev/null
+++ b/Week_02/id_55/LeetCode_242_55.py
@@ -0,0 +1,12 @@
+# 上周有个相同题目用数组但是没法处理unicode，换成字典就可以了
+    def isAnagramUnicode(self, s: str, t: str) -> bool:
+        if len(s) != len(t):
+            return False
+
+        alpha = {}
+        beta = {}
+        for c in s:
+            alpha[c] = alpha.get(c, 0) + 1
+        for c in t:
+            beta[c] = beta.get(c, 0) + 1
+        return alpha == beta
\ No newline at end of file
diff --git a/Week_02/id_55/LeetCode_441_55.py b/Week_02/id_55/LeetCode_441_55.py
new file mode 100644
index 00000000..e323f2b9
--- /dev/null
+++ b/Week_02/id_55/LeetCode_441_55.py
@@ -0,0 +1,20 @@
+# 这是上周的题一直就想到这个解，感觉拿不出手
+class Solution:
+    def arrangeCoins(self, n: int) -> int:
+        level = 1
+        while level <= n:
+            n -= level
+            level += 1
+        return level-1
+
+    def arrangeCoins2(self, n: int) -> int:
+        level = 1
+        m = int(n/2)+1 #细想了想最多有int(n/2)+1行了吧
+        while level <= m:
+            n -= level
+            if n < 0:
+                break
+            level += 1
+        return level-1
+
+    # 看了别人的二分法思路思路O(logn)
diff --git a/Week_02/id_55/LeetCode_671_55.py b/Week_02/id_55/LeetCode_671_55.py
new file mode 100644
index 00000000..c76daa1f
--- /dev/null
+++ b/Week_02/id_55/LeetCode_671_55.py
@@ -0,0 +1,27 @@
+class TreeNode:
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+
+class Solution:
+
+    def findSecondMinimumValue(self, root: TreeNode) -> int:
+        val_set = set([])
+
+        def findRun(root):
+            if not root:
+                return
+            val_set.add(root.val)
+            if (root.left):
+                findRun(root.left)
+            if (root.right):
+                findRun(root.right)
+
+        findRun(root)
+        a = list(val_set)
+        a.sort()
+        if len(a) < 2:
+            return -1
+        return a[1]
\ No newline at end of file
diff --git a/Week_02/id_55/LeetCode_783_55.py b/Week_02/id_55/LeetCode_783_55.py
new file mode 100644
index 00000000..918a5115
--- /dev/null
+++ b/Week_02/id_55/LeetCode_783_55.py
@@ -0,0 +1,22 @@
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+class Solution:
+    def minDiffInBST(self, root: TreeNode) -> int:
+        self.prev = None
+        self.min_val = float('inf')
+        self.inOrder(root)
+        return self.min_val
+
+    def inOrder(self, root):
+        if not root:
+            return
+        self.inOrder(root.left)
+        if self.prev:
+            self.min_val = min(self.min_val, root.val - self.prev.val)
+        self.prev = root
+        self.inOrder(root.right)
\ No newline at end of file
diff --git a/Week_03/id_55/LeetCode_104.55.py b/Week_03/id_55/LeetCode_104.55.py
new file mode 100644
index 00000000..6ca074a7
--- /dev/null
+++ b/Week_03/id_55/LeetCode_104.55.py
@@ -0,0 +1,42 @@
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+
+class Solution:
+    # 深度优先搜索DFS递归解法：
+    def maxDepthDFS(self, root: TreeNode) -> int:
+        # print("root val:%d" % (root.val))
+        print("root:%s" % (root))
+        if root == None:  # 递归边界
+            return 0
+        else:
+            depth_l = self.maxDepthDFS(root.left)
+            if depth_l:
+                print("depth_l:%d" % (depth_l))
+            depth_r = self.maxDepthDFS(root.right)
+            if depth_r:
+                print("depth_r:%d" % (depth_r))
+            return max(depth_l, depth_r) + 1
+    
+    def maxDepthBFS(self, root):
+        stack = []
+        if root is not None:
+            stack.append((1, root))  # 如果root不是空就把root加入stack，深度为1
+
+        depth = 0
+        while stack != []:  # 开始循环stack
+            print("stack before pop :%s" % (stack))  # 打印pop前的栈
+            current_depth, root = stack.pop()  # 先取出最后放进去的
+            print("current_depth:%d" % (current_depth))  # 打印当前深度从1开始
+            print("stack after pop :%s" % (stack))  # 打印pop后的栈
+            if root:
+                print("root.val:%d" % (root.val))
+            if root is not None:
+                depth = max(depth, current_depth)
+                stack.append((current_depth + 1, root.left))
+                stack.append((current_depth + 1, root.right))
+        return depth
\ No newline at end of file
diff --git a/Week_03/id_55/LeetCode_429_55.py b/Week_03/id_55/LeetCode_429_55.py
new file mode 100644
index 00000000..5de960f0
--- /dev/null
+++ b/Week_03/id_55/LeetCode_429_55.py
@@ -0,0 +1,24 @@
+# Definition for a Node.
+
+
+class Node:
+    def __init__(self, val, children):
+        self.val = val
+        self.children = children
+
+
+class Solution:
+    def levelOrder(self, root: 'Node') -> List[List[int]]:
+        level = []
+        if not root:
+            return []
+        level.append((0, root))
+        res = []
+        while level:
+            depth, root = level.pop(0)
+            if depth >= len(res):
+                res.append([])
+            res[depth].append(root.val)
+            for node in root.children:
+                level.append((depth+1, node))
+        return res
\ No newline at end of file
diff --git a/Week_04/.DS_Store b/Week_04/.DS_Store
new file mode 100644
index 00000000..a7b7b483
Binary files /dev/null and b/Week_04/.DS_Store differ
diff --git a/Week_04/id_55/LeetCode_169_55.py b/Week_04/id_55/LeetCode_169_55.py
new file mode 100644
index 00000000..22c393e3
--- /dev/null
+++ b/Week_04/id_55/LeetCode_169_55.py
@@ -0,0 +1,45 @@
+# coding=UTF8
+# 给定一个大小为 n 的数组，找到其中的众数。众数是指在数组中出现次数大于 ⌊ n/2 ⌋ 的元素。
+
+# 你可以假设数组是非空的，并且给定的数组总是存在众数。
+
+# 示例 1:
+
+# 输入: [3,2,3]
+# 输出: 3
+# 示例 2:
+
+# 输入: [2,2,1,1,1,2,2]
+# 输出: 2
+
+
+from collections import Counter
+
+
+class Solution:
+    def majorityElement(self, nums):
+        majority = int(len(nums)/2)
+        print(majority)
+        count_dict = {}
+        theBiggest = ''
+        for num in nums:
+            if num in count_dict.keys():
+                a = count_dict[num]
+                count_dict[num] = a + 1
+            else:
+                count_dict[num] = 1
+        print(count_dict)
+        for key, value in count_dict.items():
+            if value > majority:
+                theBiggest = key
+        return theBiggest
+
+# 学一波counter的用法...
+    def majorityElement2(self, nums):
+        c = Counter(nums)
+        res = c.most_common(1)[0][0]
+        return res
+
+words = [1, 2, 2, 2, 1, 1, 1, 2, 2]
+demo = Solution()
+print("result: %s" % demo.majorityElement3(words))
diff --git a/Week_04/id_55/LeetCode_455_55.py b/Week_04/id_55/LeetCode_455_55.py
new file mode 100644
index 00000000..ff7a0b6a
--- /dev/null
+++ b/Week_04/id_55/LeetCode_455_55.py
@@ -0,0 +1,66 @@
+# coding=UTF8
+# ---------------------
+# 假设你是一位很棒的家长，想要给你的孩子们一些小饼干。但是，每个孩子最多只能给一块饼干。
+# 对每个孩子 i ，都有一个胃口值 gi ，这是能让孩子们满足胃口的饼干的最小尺寸；并且每块饼干 j ，
+# 都有一个尺寸 sj 。如果 sj >= gi ，我们可以将这个饼干 j 分配给孩子 i ，这个孩子会得到满足。你的目标是尽可能满足越多数量的孩子，并输出这个最大数值。
+# 注意：
+# 你可以假设胃口值为正。
+# 一个小朋友最多只能拥有一块饼干。
+# 示例 1:
+# 输入: [1,2,3], [1,1]
+# 输出: 1
+# 解释:
+# 你有三个孩子和两块小饼干，3个孩子的胃口值分别是：1,2,3。
+# 虽然你有两块小饼干，由于他们的尺寸都是1，你只能让胃口值是1的孩子满足。
+# 所以你应该输出1。
+# 示例 2:
+# 输入: [1,2], [1,2,3]
+# 输出: 2
+# 解释:
+# 你有两个孩子和三块小饼干，2个孩子的胃口值分别是1,2。
+# 你拥有的饼干数量和尺寸都足以让所有孩子满足。
+# 所以你应该输出2.
+
+
+class Solution:
+    # 执行用时 : 2564 ms, 在Assign Cookies的Python3提交中击败了5.08% 的用户
+    # 内存消耗 : 14.7 MB, 在Assign Cookies的Python3提交中击败了17.98% 的用户
+    def findContentChildren(self, g, s):
+        g = sorted(g)  # 胃口
+        s = sorted(s)  # 饼干
+        num, ss, gg = 0, 0, 0
+        for i in range(ss, len(s)):
+            for j in range(gg, len(g)):
+                if s[i] >= g[j]:
+                    num += 1
+                    gg = j + 1
+                    break
+                else:
+                    ss = i + 1
+                    continue
+        return num
+
+    # 用while改造了一下居然提高这么多
+    # 执行用时 : 108 ms, 在Assign Cookies的Python3提交中击败了50.07% 的用户
+    # 内存消耗 : 14.4 MB, 在Assign Cookies的Python3提交中击败了87.93% 的用户
+    def findContentChildren(self, g, s):
+        g.sort()
+        s.sort()
+        count = 0
+        i = 0
+        j = 0
+        while i < len(g) and j < len(s):
+            if g[i] <= s[j]:
+                count += 1
+                i += 1
+                j += 1
+            else:
+                j += 1
+        return count
+
+s = [1, 1]  # 饼干
+g = [1, 2, 3]  # 孩子
+
+demo = Solution()
+print("result: %s" % demo.findContentChildren3(g, s))
+
diff --git a/Week_04/id_55/LeetCode_720_55.py b/Week_04/id_55/LeetCode_720_55.py
new file mode 100644
index 00000000..ab9bb148
--- /dev/null
+++ b/Week_04/id_55/LeetCode_720_55.py
@@ -0,0 +1,38 @@
+class Solution:
+    # 暴力方法破解，暴力遍历每个字符串前缀是不是都在数组中，在比较是不是最长的、如果等长看是不是字母序更小
+    # 开始没用set，直接用words List结果超时，果然差别很大，Set居然是O(1)
+    def longestWord(self, words):
+        wordSet = set(words)
+        theWord = ""
+        for w in words:
+            print(w)
+            isIn = True
+            for i in range(1, len(w)):
+                if w[:i] not in wordSet:
+                    print("%s is not in " % w[:i])
+                    isIn = False
+                    break
+            if isIn:
+                if not theWord or len(theWord) < len(w):
+                    theWord = w
+                elif len(w) == len(theWord) and theWord > w:
+                    theWord = w
+        return theWord
+
+    def longestWord2(self, words):
+        words.sort()  # sort成树状结构
+        res = set([''])  # 建立一个空的set 直接去重了
+        longestWord = ''
+        for word in words:
+            print(word)
+            if word[:-1] in res:  # 判断每个单词的除去最后一个字母是否在set里
+                res.add(word)
+                print(res)
+                if len(word) > len(longestWord):
+                    longestWord = word  # 保存最长的词
+        return longestWord
+
+
+words = ["a", "banana", "app", "appl", "ap", "apply", "apple"]
+demo = Solution()
+print("result: %s" % demo.longestWord(words))