[C + +] leetcode:83 combinations (backtracking method)

Topic:

Given-integers n and K, return all possible combinations of K numbers out of 1 ... n.

For example,
If N = 4 and k = 2, a solution is:

[  [2,4],  [3,4], [  2,3],  [up],  [1,3],  [1,4],]

Thought: Backtracking method

Constructs an auxiliary function, Combine_helper.

(1) Recursion once, fill in a number

(2) Fill in the number, cannot be less than the current number of values, to prevent duplication

(3) Backtracking: Pop_back () The last number added, back to the previous level

(4) The end condition, when the number is filled with k number, the current completed, backtracking.

About the specific implementation of backtracking, this article has an animation, explained very vivid: backtracking

Attention:

1. Note that the number we fill in must be incremented, otherwise it will result in duplicate numbers.

2. Grasp the essence of retrospective thinking. Pop_back is critical. Go back to the previous level.

Complexity: non-deterministic polynomial time complexity class, NP problem

AC Code:

Class Solution {public:    vector<vector<int> > Combine (int n, int k) {        vector<vector<int> > ret;        if (n = = 0 | | k = = 0 | | n < k) return ret;        vector<int> tmp;        Combine_helper (0, 0, N, K, TMP, ret);        return ret;    } Private:    void Combine_helper (int start, int num, int n, int k, vector<int> tmp, vector<vector<int> > & ret)    {        if (num = = k)        {            ret.push_back (TMP);            return;        }                for (int i = start; i < n; i++)        {            tmp.push_back (i+1);            Combine_helper (i+1, Num+1, N, K, TMP, ret);            Tmp.pop_back ();}}    ;

[C + +] leetcode:83 combinations (backtracking method)