28. Permutations FROM Leetcode

题目

Given a collection of distinct numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:

[
[1,2,3],
[1,3,2],
[2,1,3],
[2,3,1],
[3,1,2],
[3,2,1]
]

频度： 4

解题之法

class Solution {
public:
    vector > permute(vector &num) {
        vector > result;
        
        permuteRecursive(num, 0, result);
        return result;
    }
    
    // permute num[begin..end]
    // invariant: num[0..begin-1] have been fixed/permuted
    void permuteRecursive(vector &num, int begin, vector > &result)    {
        if (begin >= num.size()) {
            // one permutation instance
            result.push_back(num);
            return;
        }
        
        for (int i = begin; i < num.size(); i++) {
            swap(num[begin], num[i]);
            permuteRecursive(num, begin + 1, result);
            // reset
            swap(num[begin], num[i]);
        }
    }
};