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| 1 | +# 15. 三数之和 |
| 2 | + |
| 3 | + |
| 4 | +[url](https://leetcode-cn.com/problems/3sum/) |
| 5 | + |
| 6 | +## 题目 |
| 7 | + |
| 8 | +给你一个包含 n 个整数的数组 nums,判断 nums 中是否存在三个元素 a,b,c ,使得 a + b + c = 0 ?请你找出所有和为 0 且不重复的三元组。 |
| 9 | + |
| 10 | +注意:答案中不可以包含重复的三元组。 |
| 11 | + |
| 12 | + |
| 13 | +``` |
| 14 | +输入:nums = [-1,0,1,2,-1,-4] |
| 15 | +输出:[[-1,-1,2],[-1,0,1]] |
| 16 | +输入:nums = [] |
| 17 | +输出:[] |
| 18 | +输入:nums = [0] |
| 19 | +输出:[] |
| 20 | +``` |
| 21 | + |
| 22 | +## 方法 |
| 23 | + |
| 24 | + |
| 25 | +## code |
| 26 | + |
| 27 | +### js |
| 28 | + |
| 29 | +```js |
| 30 | +let treeSum = nums => { |
| 31 | + let ret = []; |
| 32 | + if (nums === null || nums.length === 0) |
| 33 | + return ret; |
| 34 | + nums.sort(); |
| 35 | + for (let i = 0; i < nums.length - 2; i++) { |
| 36 | + if (nums[i] > 0) |
| 37 | + break; |
| 38 | + if (i !== 0 && nums[i] === nums[i - 1]) |
| 39 | + continue; |
| 40 | + let l = i + 1, r = nums.length - 1; |
| 41 | + while (l < r) { |
| 42 | + let sum = nums[l] + nums[r]; |
| 43 | + if (sum < -nums[i]) |
| 44 | + l++; |
| 45 | + else if (sum > -nums[i]) |
| 46 | + r--; |
| 47 | + else { |
| 48 | + ret.push([nums[i], nums[l], nums[r]]); |
| 49 | + // 防止重复 |
| 50 | + while (l < r && nums[l] === nums[l + 1]) l++; |
| 51 | + while (l < r && nums[r] === nums[r - 1]) r--; |
| 52 | + l++; |
| 53 | + r--; |
| 54 | + } |
| 55 | + } |
| 56 | + } |
| 57 | + return ret; |
| 58 | +}; |
| 59 | +``` |
| 60 | + |
| 61 | +### go |
| 62 | + |
| 63 | +```go |
| 64 | +func threeSum(nums []int) [][]int { |
| 65 | + res := make([][]int, 0) |
| 66 | + if nums == nil || len(nums) == 0 { |
| 67 | + return res |
| 68 | + } |
| 69 | + sort.Ints(nums) |
| 70 | + for i := 0; i < len(nums)-2; i++ { |
| 71 | + if nums[i] > 0 { |
| 72 | + break |
| 73 | + } |
| 74 | + if i != 0 && nums[i] == nums[i-1] { |
| 75 | + continue |
| 76 | + } |
| 77 | + l, r := i + 1, len(nums) - 1 |
| 78 | + for l < r { |
| 79 | + sum := nums[l] + nums[r] |
| 80 | + if sum < -nums[i] { |
| 81 | + l++ |
| 82 | + } else if sum > -nums[i] { |
| 83 | + r-- |
| 84 | + } else { |
| 85 | + res = append(res, []int{nums[i], nums[l], nums[r]}) |
| 86 | + for l < r && nums[l] == nums[l+1] { |
| 87 | + l++ |
| 88 | + } |
| 89 | + for l < r && nums[r] == nums[r-1] { |
| 90 | + r-- |
| 91 | + } |
| 92 | + l++ |
| 93 | + r-- |
| 94 | + } |
| 95 | + } |
| 96 | + } |
| 97 | + return res |
| 98 | +} |
| 99 | +``` |
| 100 | + |
| 101 | +### java |
| 102 | + |
| 103 | +```java |
| 104 | +class Solution { |
| 105 | + public List<List<Integer>> threeSum(int[] nums) { |
| 106 | + List<List<Integer>> ret = new ArrayList<>(); |
| 107 | + if (nums == null || nums.length == 0) |
| 108 | + return ret; |
| 109 | + Arrays.sort(nums); |
| 110 | + for (int i = 0; i < nums.length - 2; i++){ |
| 111 | + if (nums[i] > 0) |
| 112 | + break; |
| 113 | + if (i != 0 && nums[i] == nums[i - 1]) |
| 114 | + continue; |
| 115 | + int l = i + 1, r = nums.length - 1; |
| 116 | + while (l < r){ |
| 117 | + int sum = nums[l] + nums[r]; |
| 118 | + if (sum < -nums[i]) |
| 119 | + l++; |
| 120 | + else if (sum > -nums[i]) |
| 121 | + r--; |
| 122 | + else { |
| 123 | + ret.add(Arrays.asList(nums[i], nums[l], nums[r])); |
| 124 | + // 防止重复 |
| 125 | + while (l < r && nums[l] == nums[l + 1]) l++; |
| 126 | + while (l < r && nums[r] == nums[r - 1]) r--; |
| 127 | + l++; |
| 128 | + r--; |
| 129 | + } |
| 130 | + } |
| 131 | + } |
| 132 | + return ret; |
| 133 | + } |
| 134 | +} |
| 135 | +``` |
| 136 | + |
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