"Sword Point offer" topic 36 reverse order in the array

Any of the two numbers in the array, if the preceding number is greater than the number that follows, is called an inverse

For example: 1,2,1,2,1 has 3 reverse

Idea: Knowing O (N2) must be wrong. Start to think hash, try to find O (n) algorithm, think for a long time, can not find. Later thought of the sort, with the original array and the array of arrays compared, I write the quick row, or not. Thought for a few hours, helpless to see the answer, the original is a transformation of the merger sort. The sort is so profound, I can't think of it in a different way ....

On the cattle online AC code: The brain is not awake, a variety of small mistakes, submitted many times.

classSolution { Public:    voidMymergesort (vector<int> &data,int& ans,intLintR, vector<int> &v) {if(L >= R)return; intm = L + (r-l)/2;        Mymergesort (data, ans, l, M, v); Mymergesort (data, ans, M+1, R, v);              Mymerge (data, ans, L, M, R, v); }    voidMymerge (vector<int> &data,int& ans,intLintMintR, vector<int> &v) {intN1 = M-l +1; intK =0, i = l, j = m +1;  while(I <= m && J <=r) {if(Data[i] <=Data[j]) {V[k+ +] = data[i++]; }            Else{v[k+ +] = data[j++]; Ans+ = M-i +1; }        }         while(I <=m) v[k+ +] = data[i++];  while(J <=r) V[k+ +] = data[j++];  for(k =0; K < R-l +1; k++) Data[k+ L] =V[k]; }    intInversepairs (vector<int>data) {        intAns =0; Vector<int>V (data.size ()); Mymergesort (data, ans,0, Data.size ()-1, V); returnans; }};

"Sword Point offer" topic 36 reverse order in the array