[LeetCode] 39. Combination Sum ☆☆☆(数组相加等于指定的数)

https://leetcode.wang/leetCode-39-Combination-Sum.html

描述

Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

The same repeated number may be chosen from candidates unlimited number of times.

Note:

All numbers (including target) will be positive integers.
The solution set must not contain duplicate combinations.
Example 1:

Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
[7],
[2,2,3]
]
Example 2:

Input: candidates = [2,3,5], target = 8,
A solution set is:
[
[2,2,2,2],
[2,3,3],
[3,5]
]

解析

回溯法

参考这里) ，就是先向前列举所有情况，得到一个解或者走不通的时候就回溯。和37题有异曲同工之处，也算是回溯法很典型的应用，直接看代码吧。

动态规划

看原文

代码

回溯法

public List<List<Integer>> combinationSum(int[] nums, int target) {
    List<List<Integer>> list = new ArrayList<>();
    backtrack(list, new ArrayList<>(), nums, target, 0);
    return list;
}

private void backtrack(List<List<Integer>> list, List<Integer> tempList, int [] nums, int remain, int start){
    if(remain < 0) return;
    else if(remain == 0) list.add(new ArrayList<>(tempList));
    else{
        for(int i = start; i < nums.length; i++){
            tempList.add(nums[i]);
            backtrack(list, tempList, nums, remain - nums[i], i);
            //找到了一个解或者 remain < 0 了，将当前数字移除，然后继续尝试
            tempList.remove(tempList.size() - 1);
        }
    }
}