Study Plan For Algorithms - Part9
1. 电话号码的字母组合
给定一个仅包含数字 2-9 的字符串,返回所有它能表示的字母组合。
class Solution:
def letterCombinations(self, digits: str) -> List[str]:
mapping = {
'2': 'abc',
'3': 'def',
'4': 'ghi',
'5': 'jkl',
'6':'mno',
'7': 'pqrs',
'8': 'tuv',
'9': 'wxyz'
}
res = []
if not digits:
return res
def backtrack(index, cur_str):
if index == len(digits):
res.append(cur_str)
return
digit = digits[index]
letters = mapping[digit]
for letter in letters:
backtrack(index + 1, cur_str + letter)
backtrack(0, '')
return res
2. 四数之和
给定一个由 n 个整数组成的数组 nums ,和一个目标值 target 。请找出并返回满足下述全部条件且不重复的四元组 [nums[a], nums[b], nums[c], nums[d]] (若两个四元组元素一一对应,则认为两个四元组重复):
- 0 <= a, b, c, d < n
- a、b、c 和 d 互不相同
- nums[a] + nums[b] + nums[c] + nums[d] == target
class Solution:
def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
nums.sort()
res = []
n = len(nums)
for i in range(n - 3):
if i > 0 and nums[i] == nums[i - 1]:
continue
for j in range(i + 1, n - 2):
if j > i + 1 and nums[j] == nums[j - 1]:
continue
left = j + 1
right = n - 1
while left < right:
total = nums[i] + nums[j] + nums[left] + nums[right]
if total == target:
res.append([nums[i], nums[j], nums[left], nums[right]])
while left < right and nums[left] == nums[left + 1]:
left += 1
while left < right and nums[right] == nums[right - 1]:
right -= 1
left += 1
right -= 1
elif total < target:
left += 1
else:
right -= 1
return res
本文来自博客园,作者:WindMay,转载请注明原文链接:https://www.cnblogs.com/stephenxiong001/p/18375096