3Sum

Description

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: 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]
]

在给定数组中找出“和为0的三个数”的所有解。

需要避免重复的解（相同数字组合的解为重复）。

Solution

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func threeSum(nums []int) (res [][]int) {
    if len(nums) < 3 {
        return
    }

    // 从小到大排序
    sort.Ints(nums)

    for i := 0; i < len(nums)-2; i++ {
        left, right := i+1, len(nums)-1
        num0 := nums[i]
        num1 := nums[left]
        num2 := nums[right]

        // 所有数同符号则不需要继续
        if (num0 > 0 && num1 > 0) ||
            (num0 < 0 && num2 < 0) {
            break
        }

        for left < right {
            // 求和
            sum := num0 + num1 + num2

            if sum == 0 {
                // 记录满足条件的解
                res = append(res, []int{num0, num1, num2})

                // 跳过相同的数字避免重复解
                for left < right && num1 == nums[left] {
                    left++
                }

                for right < left && num2 == nums[right] {
                    right--
                }
            } else if sum < 0 {
                // 和为负数则增加num1
                left++
            } else {
                // 和为正数则减小num2
                right--
            }

            num1 = nums[left]
            num2 = nums[right]
        }

        // 跳过相同的num0避免重复解
        for i < len(nums)-2 && num0 == nums[i+1] {
            i++
        }
    }

    return res
}

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