Container With Most Water @ LeetCode (Python)

4Sum @ LeetCode (Python)

kitt posted @ 2014年4月24日 01:59 in LeetCode , 5856 阅读

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 []
        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)]
                    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 ]
zuoyuan 说:
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。

kitt 说:
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

kitt 说:
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

zuoyuan 说:
2014年4月24日 02:45


cold 说:
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
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
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:
if nextWord in dddict:
if nextWord in ddd:
else:ddd[nextWord] = [pre]
if nextWord not in queue2:

for tt in queue2:
del dddict[tt]
if len(lst)!=0:#最短路径在lst非0时出现
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
return Solution.ret

def visit(self,root,onepath,start,d):
if root ==start:
elif root is not None:
for x in d[root]:


zuoyuan 说:
2014年5月09日 04:27
@cold: 我的代码供参考。

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