【代码随想录训练营】【Day 27 and 28】【回溯1-2】| Leetcode 77, 216, 17

需强化知识点

组合问题：感受遍历的横向和纵向

题目

77. 组合

注意path要深拷贝

class Solution:

    def combine(self, n: int, k: int) -> List[List[int]]:
        result = []

        def backtracking(n, k, startIndex, path, result):
            if len(path) == k:
                result.append(path[:])
                return
            
            for i in range(startIndex, n+1):
                path.append(i)
                backtracking(n, k, i+1, path, result)
                path.pop()
        
        backtracking(n, k, 1, [], result)
        return result

216. 组合总和 III

class Solution:
    def combinationSum3(self, k: int, n: int) -> List[List[int]]:
        def backtracking(k, n, start_index, path_sum, path, result):
            if len(path) == k and path_sum == n:
                result.append(path[:])
                return

            for i in range(start_index, 10):
                path_sum += i
                if path_sum > n:
                    return
                path.append(i)
                backtracking(k, n, i+1, path_sum, path, result)
                path_sum -= i
                path.pop()
        result = []
        backtracking(k, n, 1, 0, [], result)
        return result

17. 电话号码的字母组合

二刷又有些忘了：数字遍历是纵向遍历（startIndex控制），字母遍历是横向遍历（for循环）
字符串，加： +=，减：str = str[:-1]

class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        letter_dict = {2:"abc", 3:"def", 4:"ghi", 5:"jkl", 6:"mno", 7:"pqrs", 8:"tuv", 9:"wxyz"}
        result = []

        if len(digits) == 0:
            return [ ]
        def backtracking(digits, letter_dict, start_index, path, result):
            if len(path) == len(digits):
                result.append("".join(path))
                return

            letters = letter_dict[int(digits[start_index])]
            for letter in letters:
                path.append(letter)
                backtracking(digits, letter_dict, start_index+1, path, result)
                path.pop()
        backtracking(digits, letter_dict, 0, [], result)
        return result