1. 144. Binary Tree Preorder Traversal

    # 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 preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        queue = [root]
        result = []
        while queue:
            node = queue.pop()
            result.append(node ...
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  2. 143. Reorder List

    # Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def reorderList(self, head):
        """
        :type head: ListNode
        :rtype: void Do not return anything, modify head in-place instead.
        """
        if not head:
            return None
        slow = head
        fast = head
        while fast and fast ...
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  3. 142. Linked List Cycle II

    # Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """

        if not head or not head.next:
            return
        slow = head
        fast = head
        slow = slow.next
        fast = fast.next.next
        while fast ...
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  4. 141. Linked List Cycle

    # Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if not head or not head.next: # 0 or 1 item
            return False
        slow = head
        fast = head
        # >2 item, omit the ...
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  5. 139. Word Break

    class Solution(object):
    def wordBreak(self, s, wordDict):
        """
        :type s: str
        :type wordDict: List[str]
        :rtype: bool
        """

        if s == '':
            return True
        checklist = [False]*(len(s)+1)
        checklist[len(s)] = True
        for i in range(len(s)-1,-1,-1):
            for j in range(i,len(s)):
                if s[i:j ...
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  6. 138. Copy List with Random Pointer

    # -*- coding:utf-8 -*-
# Definition for singly-linked list with a random pointer.
# class RandomListNode(object):
#     def __init__(self, x):
#         self.label = x
#         self.next = None
#         self.random = None

class Solution(object):
    def copyRandomList(self, head):
        """
        :type head: RandomListNode
        :rtype: RandomListNode
        """

        if head == None: return None
        tmp = head
        while tmp:         # 首先，在原链表的每个节点后面都插入一个新节点，新节点的内容和前面的节点一样 ...
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  7. 136. Single Number

    class Solution(object):
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        ans = nums[0]
        for i in range(1, len(nums)):
            ans = ans ^ nums[i]   # x^x=0, x^0=x
        return ans
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  8. 134. Gas Station

    class Solution(object):
    def canCompleteCircuit(self, gas, cost):
        """
        :type gas: List[int]
        :type cost: List[int]
        :rtype: int
        """
        sum = 0 # total remained gas
        subsum = 0 # remained gas for each period
        index = 0
        for i in range(len(gas)):
            if subsum + gas[i] - cost[i] >= 0: # can come to next station ...
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  9. 133. Clone Graph

    # Definition for a undirected graph node
# class UndirectedGraphNode:
#     def __init__(self, x):
#         self.label = x
#         self.neighbors = []

class Solution:
    # @param node, a undirected graph node
    # @return a undirected graph node
    def cloneGraph(self, node):
        return self.clone(node, dict={})

    def clone(self, node, dict):
        if node == None:
            return None
        if ...
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  10. 131. Palindrome Partitioning

    class Solution(object):
    def partition(self, s):
        """
        :type s: str
        :rtype: List[List[str]]
        """
        if s == "":
            return []
        result = []
        length = len(s)
        for i in range(len(s)):
            self.dfs(s[:i + 1], s[i + 1:], length, result, tmp=[])
        return result

    def dfs(self, before, after, length, result, tmp):
        if self ...
    read more

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