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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:
思路：以下解法，固定两个数，用二分法查找第三个数，时间复杂度O(n2logn)，结果超时
class Solution { public: vector<vector<int>> threeSum(vector<int>& nums) { vector<vector<int>> result; vector<int> item; int i, j, target; int size = nums.size(); if(size < 3) return result; sort(nums.begin(), nums.end()); for(i = 0; i < size-2; i++){ for(j = i+1; j < size-1; j++){ target = 0-nums[i]-nums[j]; if(binarySearch(nums, j+1, size-1, target)){ item.clear(); item.push_back(nums[i]); item.push_back(nums[j]); item.push_back(target); result.push_back(item); } } } return result; } bool binarySearch(vector<int>& nums, int start, int end, int target){ if(start == end) return (nums[start]==target?true:false); int mid = (start + end) >> 1; if(nums[mid] == target) return true; else if(nums[mid] < target) return binarySearch(nums, mid+1, end, target); else return binarySearch(nums, start, mid, target); } };
以下方法OK，注意要跳过重复的数字
class Solution { public: vector<vector<int>> threeSum(vector<int>& nums) { int size = nums.size(); if(size < 3) return result; sort(nums.begin(), nums.end()); find(nums, 1, size-1, -nums[0]); for(int i = 1; i < size-2; i++){ if(nums[i]!=nums[i-1]) find(nums, i+1, size-1, -nums[i]); } return result; } void find(vector<int>& nums, int start, int end, int target){ if(start >= end) return; int sum = nums[start]+nums[end]; if(sum == target){ item.clear(); item.push_back(-target); item.push_back(nums[start]); item.push_back(nums[end]); result.push_back(item); start++; while(start!= end && nums[start] == nums[start-1]) start++; if(start == end) return; end--; while(end!=start && nums[end] == nums[end+1]) end--; if(end == start) return; find(nums, start, end, target); } else if(sum>target){ end--; while(end!=start && nums[end] == nums[end+1]) end--; if(end == start) return; find(nums, start, end, target); } else{ start++; while(start!= end && nums[start] == nums[start-1]) start++; if(start == end) return; find(nums, start, end, target); } } private: vector<vector<int>> result; vector<int> item; };
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原文地址：http://www.cnblogs.com/qionglouyuyu/p/4675179.html
