经典算法面试题系列 （二）——three sum

接着two sum，three sum的难度上升了不少。因为two sum的答案是唯一的，three sum的答案是多个。题目链接https://leetcode.com/problems/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]
]

其实还是像two sum，先假设已经找到第一个数，剩下的就是循环判断第二个数加上第三个数等于第一个数。当然还有一个问题，就是需要处理重复。处理重复思路是这样的，先找到所有的组合，再做处理。
1.golang 版

package main

import (
"fmt"
"sort"
)

func main() {
var nums = []int{-1, 0, 1, 2, -1, -4}
three := threeSum(nums)
fmt.Println(three)
}

func threeSum(nums []int) [][]int {
if len(nums) <= 0 {
return [][]int{}
}

if !sort.IntsAreSorted(nums) {
sort.Ints(nums)
}

var sum int
var res [][]int
for i := 0; i < len(nums); i++ {
target := -nums[i]
front := i + 1
back := len(nums) - 1

for front < back {
sum = nums[front] + nums[back]

if sum < target {
front++
} else if sum > target {
back--
} else {
var tripliet = []int{nums[i], nums[front], nums[back]}
res = append(res, tripliet)

for front < back && nums[front] == tripliet[1] {
front++
}
for front < back && nums[back] == tripliet[2] {
back--
}
}
}

for i+1 < len(nums) && nums[i+1] == nums[i] {
i++
}
}

return res
}

2.php 版

function threeSum($nums) {
if (count($nums) <= 0) {
return [[]];
}

sort($nums);
$res = [];
for($i = 0; $i < count($nums); $i++) {
$target = -$nums[$i];
$front = $i + 1;
$back = count($nums) - 1;

while($front < $back) {
$sum = $nums[$front] + $nums[$back];
if ($sum < $target) {
$front ++;
} else if ($sum > $target) {
$back --;
} else {
$triplet = [$nums[$i], $nums[$front], $nums[$back]];
$res[] = $triplet;
while($front < $back && $nums[$front] == $triplet[1]) {
$front ++;
}

while($front < $back && $nums[$back] == $triplet[2]) {
$back --;
}
}
}

while($i + 1 < count($nums) && $nums[$i + 1] == $nums[$i]) {
$i ++;
}
}

return $res;
}

$nums = [-1, 0, 1, 2, -1, -4];
$three = threeSum($nums);
var_dump($three);