LC287,一道medium,`O(n)+O(1)` 想出来很难,实现就两行
#找工
int findDuplicate(vector<int>& a) {
int i = 0, j = 0;
for (; i != j || j == 0; i = a[i], j = a[a[j]]);
for (i = 0; i != j; i = a[i], j = a[j]);
return i;
}#找工