Leetcode:Combination Sum与Combination Sum II

都是用递归的方法实现，深度优先，遍历数组。

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] 

实现代码：

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];        }    }}

Combination Sum II

 

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.
• 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 10,1,2,7,6,1,5 and target 8, 
A solution set is: 
[1, 7] 
[1, 2, 5] 
[2, 6] 
[1, 1, 6] 

实现代码：

public class Solution {    private ArrayList<ArrayList<Integer>> results;    public ArrayList<ArrayList<Integer>> combinationSum2(int[] candidates,            int target) {        if (candidates.length < 1) {            return results;        }        ArrayList<Integer> path = new ArrayList<Integer>();        java.util.Arrays.sort(candidates);        results = new ArrayList<ArrayList<Integer>> ();        combinationSumHelper(path, candidates, target, 0);        return results;    }    private void combinationSumHelper(ArrayList<Integer> path, int[] candidates, int sum, int pos) {        if (sum == 0) {            results.add(new ArrayList<Integer>(path));        }        if (pos >= candidates.length || sum < 0) {            return;        }        int prev = -1;        for (int i = pos; i < candidates.length; i++) {            if (candidates[i] != prev) {                path.add(candidates[i]);                combinationSumHelper(path, candidates, sum - candidates[i], i + 1);                prev = candidates[i];                path.remove(path.size()-1);            }        }    }}