Leetcode 52 - N-Queens II

Follow up for N-Queens problem.

Now, instead outputting board configurations, return the total number of distinct solutions.

Backtracking (recursive)
RecursiveNQueens(Q[1 .. n],r):
if r = n + 1
    print Q
else for j ← 1 to n
    legal ← True
    for i ← 1 to r − 1
        if (Q[i] = j) or (Q[i] = j + r − i) or (Q[i] = j − r + i)
        legal ← False
    if legal
        Q[r] ← j
        RecursiveNQueens(Q[1 .. n],r + 1)