LeetCode:3Sum Closest
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3Sum Closest
Total Accepted: 80994 Total Submissions: 275223 Difficulty: Medium
Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target.
Return the sum of the three integers. You may assume that each input would have exactly one solution.
For example, given array S = {-1 2 1 -4}, and target = 1. The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).
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思路:
暴力解法是三个循环i,j,k,判断(nums[i] + nums[i] + nums[k])与target的最接近值,时间复杂度O(n^3);
但是其实有许多重复的计算,就是在三个指针交叉指向的时候;
如:(i = 0,j=2,k=4)与(i=2,j=4,k=2)这些都是不必要的。
三个指针,first loop(0~n-1),每次seond(从first+1开始)和third(从n-1开始)相互逼近,这样可以去掉重复值;
时间复杂度可降到O(n^2)。
java code:
public class Solution { public int threeSumClosest(int[] nums, int target) { int n = nums.length; int closetSum = nums[0] + nums[1] + nums[n - 1]; Arrays.sort(nums); for(int first = 0; first < n-2; first++) { //不同有两个指针指向同一位置,因此是n-2 //if(first > 0 && nums[first] == nums[first-1]) continue;//去重用,这里不需要 int second = first + 1, third = n - 1; while(second < third) { int curSum = nums[first] + nums[second] + nums[third]; if(curSum==target) return target; if(Math.abs(target - curSum) < Math.abs(target - closetSum)) closetSum = curSum; if(curSum < target) second++; else third--; } } return closetSum; }}
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