[LeetCode][Python]15：3Sum

# -*- coding: utf8 -*-‘‘‘__author__ = ‘[email protected]‘

15: 3Sumhttps://oj.leetcode.com/problems/3sum/

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0?Find all unique triplets in the array which gives the sum of zero.

Note:Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)The solution set must not contain duplicate triplets.    For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:    (-1, 0, 1)    (-1, -1, 2)

===Comments by Dabay===先排序，然后从左到右固定一个数，在后边的数列中使用左右指针往中间靠拢的方法查找。

从左往右固定一个数(如果需要判断的数与之前的相等就跳过),左右两个指针往中间靠拢的时候，能够保证不遗漏。(如果找到一组数据之后，左指针继续移动到下一个数不相等的数)‘‘‘class Solution:    # @return a list of lists of length 3, [[val1,val2,val3]]    def threeSum(self, num):        if len(num) < 3:            return []        num.sort()        res = []        for i in xrange(len(num)-2):            if i>0 and num[i]==num[i-1]:                continue            target = 0 - num[i]            j, k = i+1, len(num)-1            while j < k:                if num[j] + num[k] == target:                    res.append([num[i], num[j], num[k]])                    j = j + 1                    while j<k and num[j]==num[j-1]:                        j = j + 1                elif num[j] + num[k] < target:                    j = j + 1                else:                    k = k - 1        return res

def main():    sol = Solution()    nums = [0,0,0,0,0]    solutions = sol.threeSum(nums)    for solution in solutions:        print solution

if __name__ == "__main__":    import time    start = time.clock()    main()    print "%s sec" % (time.clock() - start)