题解 | #三数之和#

import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        if (num.length < 3) {//长度小于3时返回空数组集
            ArrayList<ArrayList<Integer>> o = new ArrayList<>();
            return o;
        } else {
            ArrayList<ArrayList<Integer>> out = new ArrayList<>();

            for (int i = 0; i < num.length; i++) {//第一层循环，循环整个数组
                int numi = num[i];
                int two = 0 - numi;

                ArrayList<Integer> al = new ArrayList<>();
                for (int k = 0; k < num.length; k++) {
                    if (k != i) {
                        al.add(num[k]);
                    }
                }
                for (int j = 0; j < al.size(); j++) {//第二层循环，循环除numi之外的其他数
                    int numj = al.get(j);
                    int one = two - numj;

                    ArrayList<Integer> al0 = new ArrayList<>();
                    for (int l = 0; l < al.size(); l++) {
                        if (l != j) {
                            al0.add(al.get(l));
                        }
                    }
                    for (int c = 0; c < al0.size(); c++) {//第三层循环，循环二层的数组中除numj之外的其他数
                        ArrayList<Integer> out0 = new ArrayList<>();
                        int numc = al0.get(c);
                        if (numc == one) {
                            out0.add(numi);
                            out0.add(numc);
                            out0.add(numj);
                            out0.sort(Comparator.comparingInt(cc -> cc));//对三元组内部排序
                            if (!out.contains(out0)) {//将三元组排除重复的之后加入外层返回的数组
                                out.add(out0);
                            }

                        }
                    }
                }

            }
		  //对外层返回的数组排序，若三元组中第一个数字相等，则按第二个数字排序。。
            out.sort((cc, ff)-> {
                int no = 0;
                if (cc.get(0) > ff.get(0)) {
                    no = 1;
                } else if (cc.get(0) == ff.get(0) && cc.get(1) > ff.get(1)) {
                    no = 1;
                } else if (cc.get(0) == ff.get(0) && cc.get(1) == ff.get(1) && cc.get(2) > ff.get(2)) {
                    no = 1;
                } else {
                    no = -1;
                }
                return no;
            });
            return out;
        }

    }
}