回溯backtracking
回溯法思路的简单描述是：把问题的解空间转化成了图或者树的结构表示，然后使用深度优先搜索策略进行遍历，遍历的过程中记录和寻找所有可行解或者最优解。
基本思想类同于：
1.图的深度优先搜索
2.二叉树的后序遍历
分支限界法：广度优先搜索
思想类同于：图的广度优先遍历
二叉树的层序遍历
1. leetcode 39. Combination Sum
题目描述：给一个数组nums和一个目标数target，找出所有数组nums中的数组合和为target的组合，nums数组中的数可以
使用无限次，
Given a set of candidate numbers (C) (without duplicates) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.
The same repeated number may be chosen from C unlimited number of times.
Note:
All numbers (including target) will be positive integers.
The solution set must not contain duplicate combinations.
For example, given candidate set [2, 3, 6, 7] and target 7,
A solution set is:
[
[7],
[2, 2, 3]
]
code:
class Solution(object):
def combinationSum(self, candidates, target):
"""
:type candidates: List[int]
:type target: int
:rtype: List[List[int]]
"""
self.resList = []
candidates = sorted(candidates)
self.dfs(candidates,[],target,0)
return self.resList
def dfs(self, candidates, sublist, target, last):
if target == 0:
self.resList.append(sublist[:])
if target< candidates[0]:
return
for n in candidates:
if n > target:
return
if n < last: # not go back
continue
sublist.append(n)
self.dfs(candidates,sublist,target - n, n)
sublist.pop()
if __name__ == '__main__':
nums = [2,3,6,7]
Solution = Solution()
target = 7
print(Solution.combinationSum(nums,target))
上面的方法，DFS传参中需要传入sublist,候选集，每次需要append进当前的元素，因此最后需要pop()对数据出栈。
自己的code2: 更易懂但效率低，每次要对path求和，自然和为target则结果正确，添加到结果里，> target则return，否则继续DFS
class Solution(object):
def combinationSum(self, candidates, target):
candidates.sort()
res = []
index = 0
self.dfs(candidates,index,target,[],res)
return res
def dfs(self,candidates,index,target,path,res):
if sum(path) == target:
res.append(path)
if sum(path) > target:
return
for i in range(index,len(candidates)):
self.dfs(candidates,i,target,path+[candidates[i]],res)
更简洁的写法：摘自leetcode discuss:
class Solution(object):
def combinationSum(self, candidates, target):
res = []
candidates.sort()
self.dfs(candidates, target, 0, [], res)
return res
def dfs(self, nums, target, index, path, res):
if target < 0:
return # backtracking
if target == 0:
res.append(path)
return
for i in xrange(index, len(nums)):
self.dfs(nums, target-nums[i], i, path+[nums[i]], res)
2.leetcode 40. Combination Sum II
题目和上题leetcode 39 基本类似，唯一不同是数组中的所有元素不是无限次使用，而是有多少只能用多少次
Given a collection of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.
Each number in C may only be used once in the combination.
Note:
All numbers (including target) will be positive integers.
The solution set must not contain duplicate combinations.
For example, given candidate set [10, 1, 2, 7, 6, 1, 5] and target 8,
A solution set is:
[
[1, 7],
[1, 2, 5],
[2, 6],
[1, 1, 6]
]
code1: 完全暴力，不考虑任何优化，和leetcode 39的代码的区别只在于dfs递归里的i变成了i+1（跳过本身），同时leetcode 39中数组是没有重复元素的，leetcode 40里面可能有，所以需要判断重复，这里仅依靠一个字典d的O(1)判断
可以ac leetcode的tests，但是速度很低
class Solution(object):
def combinationSum2(self, candidates, target):
candidates.sort()
res = []
index = 0
self.d = {}
self.dfs(candidates,index,target,[],res)
return res
def dfs(self,candidates,index,target,path,res):
if target == 0 :
if tuple(path) not in self.d:
res.append(path)
self.d[tuple(path)] = 1
return
if target < 0:
return
for i in range(index,len(candidates)):
self.dfs(candidates,i + 1,target - candidates[i],path+[candidates[i]],res)
code2:
考虑优化：
- 当目前path为空，即即将添加第一个元素的时候，判断之前res里是否已有答案是该元素开头
比如target = 8, nums = [1, 1, 2, 5, 6, 7, 10]
则第一个1过完，到第二个1，path肯定已被第一个1的path所包含，就要跳过，否则
[1, 1, 6]
[1, 2, 5]
[1, 7]
[1, 2, 5]
[1, 7]
[2, 6]
可以看到出现了两次（1，2，5）和（1，7）
if i == index and nums[i] == res[-1][0]: # res[-1][0]是上一个答案的开头元素
- 当目前的下标不在开头，但是和之前的元素有重复，就直接跳过
比如 nums = [1,2,2,2,5] ,target = 5
答案会为
[1, 2, 2]
[1, 2, 2]
[1, 2, 2]
[5]
在遍历过第一个2的时候就不应该再去看后面的两个2了。
if i != index and candidates[i] == candidate[i-1]:
continue
可以看出，优化2的代码其实也包含了优化1，同时由于不会重复，没有必要再用字典d去保存已经有的答案。
因此最终代码为
class Solution(object):
def combinationSum2(self, candidates, target):
candidates.sort()
res = []
index = 0
self.dfs(candidates,index,target,[],res)
return res
def dfs(self,candidates,index,target,path,res):
if target == 0 :、
res.append(path)
return
if target < 0:
return
for i in range(index,len(candidates)):
if i != index and candidates[i] == candidates[i-1]:
continue
self.dfs(candidates,i + 1,target - candidates[i],path+[candidates[i]],res)
二维数组的深度遍历（参考leetcode17 Letter Combinations of a Phone Number）
给一个数组[['1','2','3'], ['4','5','6'], ['7','8','9']]
生成排列的组合：（顺序可以任意）
1,4,7,
1,4,8
1,4,9
1,5,7
1,5,8...
3,6,8,
3,6,9
def letter(nums):
path = ''
res = []
index = 0
dfs(nums,path,index,res)
return res
def dfs(nums,path,index,res):
if len(path) == len(nums):
res.append(path)
return
for i in nums[index]:
dfs(nums,path+i,index+1,res)
if __name__ == '__main__':
nums = [['a','b','c'],['d','e','f'],['g','h','i','x']]
# nums = ['a','']
print(letter(nums))
leetcode22 Generate Parentheses
生成所有N个括号可能的组合
Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.
For example, given n = 3, a solution set is:
[
"((()))",
"(()())",
"(())()",
"()(())",
"()()()"
]
分析：
- DFS返回正确结果条件,当前添加的path长度等于2 * n，且左边括号数量要等于右边括号
- DFS中途返回的条件：左边或者右边已经添加的次数大于n
或右边比左边添加的次数要多（如“())” 是不合法的）
方法1: 自己写的:
class Solution(object):
def generateParenthesis(self, n):
"""
:type n: int
:rtype: List[str]
"""
nums = ['(',')']
path, index, l, r = '', n, 0, 0
res = []
self.dfs(nums,path,index,res,l,r)
return res
def dfs(self,nums,path,index,res,l,r):
if l > index or r > index or r > l:
return
if len(path) == 2 * index:
res.append(path)
for j in nums:
if j == '(':
self.dfs(nums,path+j,index,res,l+1,r)
else:
self.dfs(nums,path+j,index,res,l,r+1)
方法2 leetcode discuss
l和r分别是长度为n的两个栈，一个保存左括号，一个保存右括号，分别出栈，最后两个都为空的时候为结果
如果右边比左边数量还少（已经添加入path更多，illegal），则中途退出,faster
class Solution(object):
def generateParenthesis(self, n):
"""
:type n: int
:rtype: List[str]
"""
#if not n:
# return []
path,left,right,res ='',n,n,[]
self.dfs(path,left,right,res)
return res
def dfs(self,path,left,right,res):
if right < left:
return
if not left and not right:
res.append(path[:])
return
if left > 0:
self.dfs(path + '(',left - 1,right,res)
if right > 0:
self.dfs(path + ')',left ,right - 1,res)
按照这个思路也可以给方法1中的dfs的for训练改成if
class Solution(object):
def generateParenthesis(self, n):
"""
:type n: int
:rtype: List[str]
"""
nums = ['(',')']
path, index, l, r = '', n, 0, 0
res = []
self.dfs(nums,path,index,res,l,r)
return res
def dfs(self,nums,path,index,res,l,r):
if r > l or l> index or r > index:
return
if len(path) == 2 * index :
res.append(path)
if l <= index:
self.dfs(nums,path+'(',index,res,l+1,r)
if r <= index:
self.dfs(nums,path+')',index,res,l,r+1)
全排列：
[1,2,3] 生成 123 132 213 231 312 321 六种全排列
- 网上常见的递归写法：交换与开头元素的位置
class Solution(object):
def permute(self, nums):
"""
:type nums: List[int]
:rtype: List[List[int]]
"""
end = len(nums)
begin = 0
global res
res = []
self.perm(nums,begin,end)
# res.append(perm(nums,begin,end))
return res
def perm(self,nums,begin,end):
if begin >= end:
tmp = nums[:]
res.append(tmp)
else:
for i in range(begin,end):
nums[begin],nums[i] = nums[i],nums[begin]
self.perm(nums,begin+1,end)
nums[begin],nums[i] = nums[i],nums[begin]
- DFS写法
DFS是树型结构
即第一个节点搜索1,2,3，再每个节点下继续搜索1，2，3，判断return的原则一是碰到了重复元素（相当于剪枝），二是得到正确结果
class Solution(object):
def permute(self, nums):
index,path = 0,[]
res = []
d = {}
self.dfs(nums,index,path,res,d)
def dfs(self,nums,index,path,res,d):
if len(set(path)) != len(path):
return
if len(path) == len(nums):
print(path)
return
for i in nums:
self.dfs(nums,index,path+[i],res,d)
上面的方法中需要将python中的list转化为set，比较长度来判断list是否有重复来完成剪枝，list转换set还是比较耗时的，因此另一种方法，直接在nums中去掉已经添加的元素，代码更简洁也更高效
nums = nums[:i] + nums[i+1:]
class Solution(object):
def permute(self, nums):
path = []
res = []
length = len(nums)
self.dfs(nums,path,length,res)
return res
def dfs(self,nums,path,length,res):
if len(path) == length:
res.append(path)
for i in range(len(nums)):
self.dfs(nums[:i] + nums[i+1:],path + [nums[i]],length,res)
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