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

Source: Internet
Author: User

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

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