LeetCode: Combination Sum 解题报告

Combination Sum

Combination Sum Total Accepted: 25850 Total Submissions: 96391 My Submissions Question Solution
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]

SOLUTION 1:

经典递归模板。

i 的起始值是跟排列的最主要区别。因为与顺序无关，所以我们必须只能是升序，也就是说下一个取值只能是i本身或是i的下一个。
但是排列的话，可以再取前同的。1, 2 与2 1是不同的排列，但是同一个组合

同学们可以看下这几个题目的区别。

LeetCode: Letter Combinations of a Phone Number 解题报告

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LeetCode: Permutations 解题报告

LeetCode: Combinations 解题报告

public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        if (candidates == null || candidates.length == 0) {
            return ret;
        }
        
        // Sort to avoid duplicate solutions.
        Arrays.sort(candidates);
        
        dfs(candidates, target, new ArrayList<Integer>(), ret, 0);
        return ret;
    }
    
    public void dfs(int[] candidates, int target, List<Integer> path, List<List<Integer>> ret, int index) {
        if (target < 0) {
            return;
        }
        
        if (target == 0) {
            ret.add(new ArrayList(path));
            return;
        }
        
        // i 的起始值是跟排列的最主要区别。因为与顺序无关，所以我们必须只能是升序，也就是说下一个取值只能是i本身
        // 或是i的下一个。
        // 但是排列的话，可以再取前同的。1, 2 与2 1是不同的排列，但是同一个组合
        for (int i = index; i < candidates.length; i++) {
            int num = candidates[i];
            path.add(num);
            
            // 注意，最后的参数是i，不是index!!
            dfs(candidates, target - num, path, ret, i);
            path.remove(path.size() - 1);
        }
    }
}

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/combination/CombinationSum_1203.java