Merge pull request #110 from hhf666feng/master

第二周作业#26

Author: GeekUniversity

Week_01/id_26/LeetCode_15_26.py

@@ -3,6 +3,9 @@ def threeSum(self, nums):
         """
         :type nums: List[int]
         :rtype: List[List[int]]
+        思路：
+        1) 防止重复结果，所以先排序，遍历时跳过相邻的重复值
+        2) 二分查找，大于0则右测向左移动，小于0则左边向右移动
         """
         nums.sort()
         ret, n = [], len(nums)

Week_01/id_26/LeetCode_26_26.py

@@ -6,8 +6,7 @@ def removeDuplicates(self, nums):
         """
         k = 1
         for n in nums:
-            if n != nums[k-1]:
+            if n != nums[k - 1]:
                 nums[k] = n
                 k += 1
         return k
-        

Week_02/id_26/LeetCode_101_26.py

@@ -0,0 +1,66 @@
+#
+# @lc app=leetcode.cn id=101 lang=python
+#
+# [101] 对称二叉树
+#
+# https://leetcode-cn.com/problems/symmetric-tree/description/
+#
+# algorithms
+# Easy (46.65%)
+# Likes:    351
+# Dislikes: 0
+# Total Accepted:    34.7K
+# Total Submissions: 74.4K
+# Testcase Example:  '[1,2,2,3,4,4,3]'
+#
+# 给定一个二叉树，检查它是否是镜像对称的。
+#
+# 例如，二叉树 [1,2,2,3,4,4,3] 是对称的。
+#
+# ⁠   1
+# ⁠  / \
+# ⁠ 2   2
+# ⁠/ \ / \
+# 3  4 4  3
+#
+#
+# 但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:
+#
+# ⁠   1
+# ⁠  / \
+# ⁠ 2   2
+# ⁠  \   \
+# ⁠  3    3
+#
+#
+# 说明:
+#
+# 如果你可以运用递归和迭代两种方法解决这个问题，会很加分。
+#
+#
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+
+class Solution(object):
+    def isSymmetric(self, root):
+        """
+        :type root: TreeNode
+        :rtype: bool
+        """
+
+        def _isSymmetric(node1, node2):
+            if not node1 and not node2:
+                return True
+            if not node1 or not node2:
+                return False
+            if node1.val != node2.val:
+                return False
+            return _isSymmetric(node1.left, node2.right) and _isSymmetric(
+                node1.right, node2.left)
+
+        return _isSymmetric(root, root)

Week_02/id_26/LeetCode_102_26.py

@@ -0,0 +1,65 @@
+#
+# @lc app=leetcode.cn id=102 lang=python
+#
+# [102] 二叉树的层次遍历
+#
+# https://leetcode-cn.com/problems/binary-tree-level-order-traversal/description/
+#
+# algorithms
+# Medium (56.47%)
+# Likes:    213
+# Dislikes: 0
+# Total Accepted:    28.4K
+# Total Submissions: 50.2K
+# Testcase Example:  '[3,9,20,null,null,15,7]'
+#
+# 给定一个二叉树，返回其按层次遍历的节点值。 （即逐层地，从左到右访问所有节点）。
+#
+# 例如:
+# 给定二叉树: [3,9,20,null,null,15,7],
+#
+# ⁠   3
+# ⁠  / \
+# ⁠ 9  20
+# ⁠   /  \
+# ⁠  15   7
+#
+#
+# 返回其层次遍历结果：
+#
+# [
+# ⁠ [3],
+# ⁠ [9,20],
+# ⁠ [15,7]
+# ]
+#
+#
+#
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+
+class Solution(object):
+    def levelOrder(self, root):
+        """
+        :type root: TreeNode
+        :rtype: List[List[int]]
+        """
+        if not root:
+            return []
+        ret, layer = [], [root]
+        while layer:
+            tmp_layer, line = [], []
+            for i in layer:
+                line.append(i.val)
+                if i.left:
+                    tmp_layer.append(i.left)
+                if i.right:
+                    tmp_layer.append(i.right)
+            layer = tmp_layer
+            ret.append(line)
+        return ret

Week_02/id_26/LeetCode_103_26.py

@@ -0,0 +1,68 @@
+#
+# @lc app=leetcode.cn id=103 lang=python
+#
+# [103] 二叉树的锯齿形层次遍历
+#
+# https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/description/
+#
+# algorithms
+# Medium (49.74%)
+# Likes:    64
+# Dislikes: 0
+# Total Accepted:    11.7K
+# Total Submissions: 23.5K
+# Testcase Example:  '[3,9,20,null,null,15,7]'
+#
+# 给定一个二叉树，返回其节点值的锯齿形层次遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。
+#
+# 例如：
+# 给定二叉树 [3,9,20,null,null,15,7],
+#
+# ⁠   3
+# ⁠  / \
+# ⁠ 9  20
+# ⁠   /  \
+# ⁠  15   7
+#
+#
+# 返回锯齿形层次遍历如下：
+#
+# [
+# ⁠ [3],
+# ⁠ [20,9],
+# ⁠ [15,7]
+# ]
+#
+#
+#
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+
+class Solution(object):
+    def zigzagLevelOrder(self, root):
+        """
+        思路：层序遍历，每隔一层换一个方向遍历
+        :type root: TreeNode
+        :rtype: List[List[int]]
+        """
+        if not root:
+            return []
+        line, ret, rot = [root], [], 1
+        while line:
+            tmp1, tmp2 = [], []
+            for i in line:
+                if i.left:
+                    tmp1.append(i.left)
+                if i.right:
+                    tmp1.append(i.right)
+            for j in line[::rot]:
+                tmp2.append(j.val)
+            ret.append(tmp2)
+            line = tmp1
+            rot *= -1
+        return ret

Week_02/id_26/LeetCode_111_26.py

@@ -0,0 +1,55 @@
+#
+# @lc app=leetcode.cn id=111 lang=python
+#
+# [111] 二叉树的最小深度
+#
+# https://leetcode-cn.com/problems/minimum-depth-of-binary-tree/description/
+#
+# algorithms
+# Easy (38.70%)
+# Likes:    125
+# Dislikes: 0
+# Total Accepted:    19K
+# Total Submissions: 49K
+# Testcase Example:  '[3,9,20,null,null,15,7]'
+#
+# 给定一个二叉树，找出其最小深度。
+#
+# 最小深度是从根节点到最近叶子节点的最短路径上的节点数量。
+#
+# 说明: 叶子节点是指没有子节点的节点。
+#
+# 示例:
+#
+# 给定二叉树 [3,9,20,null,null,15,7],
+#
+# ⁠   3
+# ⁠  / \
+# ⁠ 9  20
+# ⁠   /  \
+# ⁠  15   7
+#
+# 返回它的最小深度  2.
+#
+#
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+
+class Solution(object):
+    def minDepth(self, root):
+        """
+        :type root: TreeNode
+        :rtype: int
+        """
+        if not root:
+            return 0
+        left = self.minDepth(root.left)
+        right = self.minDepth(root.right)
+        if not left or not right:
+            return 1 + left + right
+        return 1 + min(left, right)

Week_02/id_26/LeetCode_1_26.py

@@ -0,0 +1,43 @@
+#
+# @lc app=leetcode.cn id=1 lang=python
+#
+# [1] 两数之和
+#
+# https://leetcode-cn.com/problems/two-sum/description/
+#
+# algorithms
+# Easy (46.10%)
+# Likes:    5365
+# Dislikes: 0
+# Total Accepted:    397.2K
+# Total Submissions: 861.4K
+# Testcase Example:  '[2,7,11,15]\n9'
+#
+# 给定一个整数数组 nums 和一个目标值 target，请你在该数组中找出和为目标值的那 两个 整数，并返回他们的数组下标。
+#
+# 你可以假设每种输入只会对应一个答案。但是，你不能重复利用这个数组中同样的元素。
+#
+# 示例:
+#
+# 给定 nums = [2, 7, 11, 15], target = 9
+#
+# 因为 nums[0] + nums[1] = 2 + 7 = 9
+# 所以返回 [0, 1]
+#
+#
+#
+
+
+class Solution(object):
+    def twoSum(self, nums, target):
+        """
+        :type nums: List[int]
+        :type target: int
+        :rtype: List[int]
+        """
+        s = {}
+        for i in range(len(nums)):
+            tmp = target - nums[i]
+            if tmp in s:
+                return [s[tmp], i]
+            s[nums[i]] = i