78. Subsets

77. Combinations

class Solution:
    def combine(self, n, k):
        res = []
        def dfs(k, tmp, start):
            if k == 0:
                res.append(tmp.copy())
                return

            for num in range(start, n + 1):
                tmp.append(num)
                dfs(k - 1, tmp, num + 1)
                tmp.pop()

        dfs(k, [], 1)
        return res

227. Basic Calculator II

class Solution:
    def combinationSum3(self, k, n):
        res = []
        def dfs(k, start, sum, tmp):
            if k == 0:
                if sum == 0:
                    res.append(tmp.copy())
                return

            for i in range(start, 10):
                tmp.append(i)
                dfs(k-1, i+1, sum-i, tmp)
                tmp.pop()
        dfs(k, 1, n, [])
        return res

    def combinationSum3_2(self, k: int, n: int) -> List[List[int]]:
        res = []
        def dfs(k, start, sum, tmp):
            if k == 0:
                if sum == 0:
                    res.append(tmp.copy())
                return

            for i in range(start, 10):
                # 剪枝
                if k > 1 and sum - i <= 0 :
                    continue
                else:
                    tmp.append(i)
                    dfs(k-1, i+1, sum-i, tmp)
                    tmp.pop()
        dfs(k, 1, n, [])
        return res

17. Letter Combinations of a Phone Number

class Solution:
    def letterCombinations(self, digits):
        if digits == '':
            return []

        dic = {'2': ['a', 'b', 'c'],
               '3': ['d', 'e', 'f'],
               '4': ['g', 'h', 'i'],
               '5': ['j', 'k', 'l'],
               '6': ['m', 'n', 'o'],
               '7': ['p', 'q', 'r', 's'],
               '8': ['t', 'u', 'v'],
               '9': ['w', 'x', 'y', 'z']}

        res = []
        def dfs(index, tmp):
            if index == len(digits):
                res.append(tmp)
                return

            for char in dic[digits[index]]:
                tmp += char
                dfs(index+1, tmp)
                tmp = tmp[:-1]
        dfs(0, '')
        return res

39. Combination Sum

class Solution:
    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
        res = []
        candidates = sorted(candidates)  # 剪枝需要
        def dfs(start, target, tmp):
            if target == 0:
                res.append(tmp.copy())
                return

            for index in range(start, len(candidates)):
           		# 剪枝
                if target - candidates[index] < 0:
                    break
                tmp.append(candidates[index])
                dfs(index, target-candidates[index], tmp)
                tmp.pop()
        dfs(0, target, [])
        return res

40. Combination Sum II

只能用一次！
- 去重trick：搜过的就不用再搜了，continue掉；ps，先sort，那么就是跟前面对比下一不一样就知道搜过没了；
- if index > start and candidates[index - 1] == candidates[index]: continue!!!重要！
- 另外剪枝了
  1. 因为只能用一次，判断下当前到最后还够不够，一个点不够了，后面的点就都不够了；break掉；
  2. 和超过target；由于sort过了，所以break掉；

class Solution:
    def combinationSum2(self, candidates, target):
        res = []
        candidates = sorted(candidates)
        def dfs(start, target, tmp):
            if target == 0:
                if tmp not in res:
                    res.append(tmp.copy())
                return

            for index in range(start, len(candidates)):
                if sum(candidates[start:]) < target:
                    break
                if target - candidates[index] < 0:
                    break
                if index > start and candidates[index - 1] == candidates[index]:
                    continue
                tmp.append(candidates[index])
                dfs(index+1, target-candidates[index], tmp)
                tmp.pop()
        dfs(0, target, [])
        return res