Permutations

Given a collection of 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], and [3,2,1].

class Solution {
public:
    void heapKeep(vector<int> &num,int i,int len){
        int max=i;
        int le = 2*i;
        int ri = 2*i+1;
        if (le<len && num[le]>num[max])
            max = le;
        if (ri<len && num[ri]>num[max])
            max = ri;
        if (max != i) {
            swap(num[i],num[max]);
            heapKeep(num,max,len);
        }
    }
    void heapBuild(vector<int> &num){
        int len = num.size();
        for (int i=len/2; i>=0; --i){
            heapKeep(num,i,len);
        }
    }
    void heapSort(vector<int> &num){
        int len = num.size();
        heapBuild(num);
        for (int i=len-1; i>0; --i){
            swap(num[0],num[i]);
            heapKeep(num,0,i);
        }
    }
    void reserveVec(vector<int> &num,int begin,int end){
        while (begin<end) {
            swap (num[begin++],num[end--]);
        }
    }
    vector<vector<int> > permute(vector<int> &num) {
        //从小到大
        int len = num.size();
        vector<vector<int>> vec;
        if (len==0) return vec;
        heapSort(num);
        while (true) {
            int index =len -1;
            int littlebig = index;
            vec.push_back(num);
            while (index>=1 && num[index-1]>num[index]) {
                index--;
            }
            if (index==0) return vec;
            while (num[littlebig]<=num[index-1]){
                littlebig--;
            }
            swap(num[littlebig],num[index-1]);
            reserveVec(num,index,len-1);
        }
        return vec;
    }
};