Sort by Color

• Time : $O(nlogn)$
• Space : $O(n)$

void dnc(vector<int>& A, int l, int r) {
    if(l == r) return;
    int m = l + (r - l) / 2;
    dnc(A,l,m);
    dnc(A,m+1,r);
    vector<int> now;
    int i = l, j = m + 1;
    while(i <= m and j <= r) {
        if(A[i] < A[j]) now.push_back(A[i++]);
        else now.push_back(A[j++]);
    }
    while(i <= m) now.push_back(A[i++]);
    while(j <= r) now.push_back(A[j++]);
    for(int i = l, j = 0; i <= r; i++,j++) {
        A[i] = now[j];
    }
}

void Solution::sortColors(vector<int> &A) {
    dnc(A,0,A.size() - 1);
}