LeetCode Solutions

Pascal's Triangle II.py

class Solution:
    from functools import lru_cache
    def getRow(self, rowIndex: int) -> List[int]:
        # 1 0  0  0 0 0    j
        # 1 1  0  0 0 0
        # 1 2  1  0 0 0
        # 1 3  3  1 0 0
        # 1 4  6  4 1 0
        # 1 5 10 10 5 1
    #   i
        
        @lru_cache()
        def row(i, j):
            if i < 0 or j < 0: return 0
            if i == 0 and j == 0: return 1
            return row(i-1, j-1) + row(i-1, j)
        
        return [row(rowIndex, i) for i in range(rowIndex+1)]
        

ReverseLinkedList(iterative).py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        if head == None: return head # len 0
        if head.next == None: return head # len 1
    
        old = None
        cur = head
        nxt = head.next
        
        # NULL    1 -> 2 -> 3 -> 4 -> 5 -> NULL
        # old    cur  next
        # NULL <- 1    2 -> 3 -> 4 -> 5 -> NULL
        #        old  cur  next
        # NULL <- 1 <- 2    3 -> 4 -> 5 -> NULL
        #             old  cur  next
        
        while nxt != None:
            cur.next = old
            old, cur, nxt = cur, nxt, nxt.next
        
        cur.next = old
        
        return cur

ReverseLinkedList(recursive).py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: ListNode, prev = None) -> ListNode:
        #         1 -> 2 -> 3 -> 4 -> 5 -> NULL
        # NULL    1 -> 2 -> 3 -> 4 -> 5 -> NULL
        # prev  head  tmp    
        
        # NULL <- 1    2 -> 3 -> 4 -> 5 -> NULL
        # NULL <- 1 <- 2    3 -> 4 -> 5 -> NULL
        # NULL <- 1 <- 2 <- 3    4 -> 5 -> NULL

        if head is None: return prev
        tmp = head.next
        head.next = prev
        
        return self.reverseList(head = tmp, prev = head)