leetcode 39: Combination Sum

Total Accepted: 32226 Total Submissions: 117691

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, … , a_k) must be in non-descending order. (ie, a₁ ≤ a₂ ≤ … ≤ a_k).
• 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]

[思路]

最常见的backtracking题目. backtracking最需要注意的是剪枝(pruning). 为了避免结果中有重复发生, 先把数列中重复的数去掉.


[CODE]

public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        TreeSet<Integer> ts = new TreeSet<Integer>();
        for(int i=0; i<candidates.length; i++) {
            ts.add(candidates[i]);
        }
        ArrayList<Integer> candi = new ArrayList<Integer>(ts);
        
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        List<Integer> cur = new ArrayList<Integer>();
        rec(res, cur, 0, target, candi, 0);
        return res;
    }
    
    private void rec( List<List<Integer>> res, List<Integer> cur, int total, int target, ArrayList<Integer> candi, int level) {
        if(total==target) {
            res.add(new ArrayList(cur));
            return;
        } else if(total>target) {
            return;
        } 
        
        for(int i=level; i<candi.size(); i++) {
            total += candi.get(i);
            cur.add(candi.get(i));
            rec(res, cur, total, target, candi, i);
            cur.remove(cur.size() - 1);
            total -= candi.get(i);
        }
    }
}