4Sum @ LeetCode (Python)
先对num排序, 然后建一个dictionary d, d[num[p]+num[q]] = [(p,q) pairs 满足num[p] + num[q]], 而且这里的(p,q) pair总是满足p < q。然后用二层循环来搜, num[i]是四元组最小的数, num[j]是第二小的数, 判断d中有没有target - num[i] - num[j]这个key的时间是O(1), 如果有这个key, 就把找到的四元组加入最后的返回结果。res使用set()来去重, 否则对于输入[-3,-2,-1,0,0,1,2,3], 0会出现两个[-3, 0, 1, 2]和两个[-2, -1, 0, 3]。
First sort num, then build a dictionary d, d[num[p]+num[q]] = [(p,q) pairs which satisfy num[p] + num[q]], here all (p,q) pairs satisfy p < q. Then use a nested for-loop to search, num[i] is the min number in quadruplet and num[j] is the second min number. The time complexity of checking whether d has the key target - num[i] - num[j] is O(1). If this key exists, add one quadruplet to the result. Use set() to remove duplicates in res, otherwise for input [-3,-2,-1,0,0,1,2,3], 0 there will be two [-3, 0, 1, 2] and two [-2, -1, 0, 3].
class Solution:
# @return a list of lists of length 4, [[val1,val2,val3,val4]]
def fourSum(self, num, target):
numLen, res, d = len(num), set(), {}
if numLen < 4: return []
num.sort()
for p in xrange(numLen):
for q in xrange(p + 1, numLen):
if num[p] + num[q] not in d:
d[ num[p] + num[q] ] = [(p,q)]
else:
d[ num[p] + num[q] ].append( (p,q) )
for i in xrange(numLen):
for j in xrange(i + 1, numLen - 2):
T = target - num[i] - num[j]
if T in d:
for k in d[T]:
if k[0] > j: res.add( ( num[i], num[j], num[k[0]], num[k[1]] ) )
return [ list(i) for i in res ]
2014年4月24日 02:03
牛逼啊，我就剩一道word ladder ii了。看来这个4sum果然还是要用hash啊。题的原意可能就是要求用hash的，直接O(n^3)应该不是原题的意思。
2014年4月24日 02:09
@zuoyuan: 可能吧，我用O(n^3)过不了，你做的好快呀
2014年4月24日 02:11
@zuoyuan: word ladder ii 我也是一直没过, 不知道有什么好的思路
2014年4月24日 02:45
各种参考你的解答，也断断续续做了两个月，一点也不快啊，呵呵。
2014年5月05日 02:32
@kitt: 参考你word ladder的代码，险过word ladder ii，应该还可以优化
@zuoyuan
class Solution:
# @param start, a string
# @param end, a string
# @param dict, a set of string
# @return a list of lists of string
def findLadders(self, start, end, dict):
lst = []#存end的前驱单词
queue1 = [start]#存当前要遍历的单词
queue2 = []#和queue1作比较，用来判断end可不可达
ddd = {}#存路径上所有单词的前驱单词，键是单词，值是前驱单词的列表
flag = True
dddict={}.fromkeys(dict)
if start in dddict:
del dddict[start]
if end in dddict:
del dddict[end]
while flag:
for pre in queue1 :
for i in xrange(len(pre)):
part1 = pre[:i]
part2 = pre[i+1:]
for j in 'abcdefghijklmnopqrstuvwxyz':
if pre[i] != j:
nextWord = part1 + j + part2
if nextWord == end:
if pre not in lst:
lst.append(pre)
else:
if nextWord in dddict:
if nextWord in ddd:
ddd[nextWord].append(pre)
else:ddd[nextWord] = [pre]
if nextWord not in queue2:
queue2.append(nextWord)
for tt in queue2:
del dddict[tt]
if len(lst)!=0:#最短路径在lst非0时出现
flag=False
if queue1==queue2:
return []
queue1 = queue2
queue2 = []
Solution.ret = []
for k in lst:#从end的前驱单词开始找路径
self.visit(k,[end],start,ddd)
return Solution.ret
def visit(self,root,onepath,start,d):
if root ==start:
onepath.append(root)
onepath.reverse()
Solution.ret.append(onepath)
elif root is not None:
onepath.append(root)
for x in d[root]:
self.visit(x,onepath[:],start,d)
return
2014年5月09日 04:27
http://www.cnblogs.com/zuoyuan/p/3697045.html
@cold: 我的代码供参考。
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