Combination Sum Leetcode

Posted 2020-09-05 璨璨要好好学习

Given a set of candidate numbers (C) (without duplicates) 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.
• 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]
]

 

This question is using dfs or backtracking. First, I did it in a very slow way.

public class Solution {
    List<List<Integer>> result;
    List<Integer> current;
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        result = new ArrayList<>();
        if (candidates == null || candidates.length == 0) {
            return result;
        }
        current = new ArrayList<>();
        helper(candidates, target);
        return result;
    }
    public void helper(int[] candidates, int target) {
        if (target == 0) {
            //Remove duplicates
            List<Integer> tmp = new ArrayList<>(current);
            Collections.sort(tmp);
            if (!result.contains(tmp)) {
                result.add(tmp);
            }
            return;
        }
        if (target < 0) {
            return;
        }
        for (int i = 0; i < candidates.length; i++) {
            current.add(candidates[i]);
            helper(candidates, target - candidates[i]);
            current.remove(current.size() - 1);
        }
    }
}

But it can be optimized a lot by sort the input first and don‘t look back.

public class Solution {
    List<List<Integer>> result;
    List<Integer> current;
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        result = new ArrayList<>();
        if (candidates == null || candidates.length == 0) {
            return result;
        }
        Arrays.sort(candidates);
        current = new ArrayList<>();
        helper(candidates, target, 0);
        return result;
    }
    public void helper(int[] candidates, int target, int start) {
        if (target == 0) {
            result.add(new ArrayList<>(current));
            return;
        }
        if (target < 0) {
            return;
        }
        for (int i = start; i < candidates.length && target >= candidates[i]; i++) {
            current.add(candidates[i]);
            helper(candidates, target - candidates[i], i);
            current.remove(current.size() - 1);
        }
    }
}

但是第二种方法在lintcode上是过不了的。如果input是[2, 2, 3]就会出现重复。如果面试遇到可以问问input会不会有重复。可以加一步判断，但没有必要排序了，因为加进去的本身就已经排序了。

我发现dfs的套路还都挺像的，注意一下有start的地方就好了。同时arraylist也可以作为参数传递。