L1：Combination Sum

https://oj.leetcode.com/problems/combination-sum/

Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T. The same repeated number may be chosen from C unlimited number of times. Note: All numbers (including target) will be positive integers. Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak). The solution set must not contain duplicate combinations. For example, given candidate set 2,3,6,7 and target 7, A solution set is: [7] [2, 2, 3]

/** * Copyright: NineChapter * - Algorithm Course, Mock Interview, Interview Questions * - More details on: http://www.ninechapter.com/ */public class Solution {    public  ArrayList<ArrayList<Integer>> combinationSum(int[] candidates, int target) {        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();        if (candidates == null) {            return result;        }        ArrayList<Integer> path = new ArrayList<Integer>();        Arrays.sort(candidates);        helper(candidates, target, path, 0, result);        return result;    }     void helper(int[] candidates, int target, ArrayList<Integer> path, int index,        ArrayList<ArrayList<Integer>> result) {        if (target == 0) {            result.add(new ArrayList<Integer>(path));            return;        }        int prev = -1;        for (int i = index; i < candidates.length; i++) {            if (candidates[i] > target) {                break;            }            if (prev != -1 && prev == candidates[i]) {                continue;            }            path.add(candidates[i]);            helper(candidates, target - candidates[i], path, i, result);            path.remove(path.size() - 1);            prev = candidates[i];        }    }}