Combination Sum II Leetcode

Given a collection of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

Each number in C may only be used once in the combination.

Note:

• All numbers (including target) will be positive integers.
• The solution set must not contain duplicate combinations.

 

For example, given candidate set [10, 1, 2, 7, 6, 1, 5] and target 8, 
A solution set is: 

[
  [1, 7],
  [1, 2, 5],
  [2, 6],
  [1, 1, 6]
]

 

 

这道题和combination sum非常像，只是需要注意，这个需要判断是不是重复的，因为input是可能有重复值的。可以用contains来判断是否有重复，但是很慢，比较妙的是添加的时候就判断了。

用contains判断

public class Solution {
    List<List<Integer>> result;
    List<Integer> current;
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        result = new ArrayList<>();
        if (candidates == null || candidates.length == 0) {
            return result;
        }
        current = new ArrayList<>();
        Arrays.sort(candidates);
        helper(candidates, target, 0);
        return result;
    }
    public void helper(int[] candidates, int target, int start) {
        if (target < 0) {
            return;
        }
        if (target == 0) {
            List<Integer> tmp = new ArrayList<>(current);
            if (!result.contains(tmp)) {
                result.add(new ArrayList<>(current));
            }
            return;
        }
        for (int i = start; i < candidates.length && target >= candidates[i]; i++) {
            current.add(candidates[i]);
            helper(candidates, target - candidates[i], i + 1);
            current.remove(current.size() - 1);
        }
    }
}

添加时判断

public class Solution {
    List<List<Integer>> result;
    List<Integer> current;
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        result = new ArrayList<>();
        if (candidates == null || candidates.length == 0) {
            return result;
        }
        current = new ArrayList<>();
        Arrays.sort(candidates);
        helper(candidates, target, 0);
        return result;
    }
    public void helper(int[] candidates, int target, int start) {
        if (target < 0) {
            return;
        }
        if (target == 0) {
            List<Integer> tmp = new ArrayList<>(current);
            result.add(new ArrayList<>(current));
            return;
        }
        for (int i = start; i < candidates.length && target >= candidates[i]; i++) {
            if (i > start && candidates[i] == candidates[i - 1]) {
                continue;
            }
            current.add(candidates[i]);
            helper(candidates, target - candidates[i], i + 1);
            current.remove(current.size() - 1);
        }
    }
}

需注意这种判断方式不适合可重复取同一个数字的情况（也就是不适合combination sum lintcode)， 因为重复取的时候start一直在左边，所以重复的数字也会被加进去，会导致重复的结果。