LeetCode

题目：

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：递归，不断地尝试余下的值。

package sum;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class CombinationSum {

    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        Arrays.sort(candidates);
        int len = removeDuplicates(candidates);
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        List<Integer> record = new ArrayList<Integer>();
        combine(candidates, 0, len, res, record, target);
        return res;
    }
    
    private void combine(int[] nums, int start, int end, List<List<Integer>> res, List<Integer> record, int target) {
        for (int i = start; i < end; ++i) {
            List<Integer> newRecord = new ArrayList<Integer>(record);
            newRecord.add(nums[i]);
            int sum = sum(newRecord);
            int rem = target - sum;
            if (rem == 0) {
                res.add(newRecord);
            } else if (rem > 0 && rem >= nums[i]) {
                combine(nums, i, end, res, newRecord, target);
            } else if (rem < 0) {
                break;
            }
        }
    }
    
    private int sum(List<Integer> record) {
        int sum = 0;
        for (int i : record)
            sum += i;
        return sum;
    }
    
    private int removeDuplicates(int[] nums) {
        int n = nums.length;
        int j = 0;
        for (int i = 1; i < n; ++i) {
            if (nums[i] != nums[i - 1]) {
                nums[++j] = nums[i];
            }
        }
        
        return j + 1;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int[] nums = {  1, 2, 2, 3, 6, 7 };
        CombinationSum c = new CombinationSum();
        List<List<Integer>> res = c.combinationSum(nums, 7);
        for (List<Integer> l : res) {
            for (int i : l)
                System.out.print(i + "	");
            System.out.println();
        }
    }

}