feat(geektime_algo): add 39 back_tracking N_queens

rust/39_back_tracking/n_queens.rb

@@ -0,0 +1,45 @@
+# leetcode 51 [N-queens])https://leetcode.com/problems/n-queens/)
+
+pub fn solve_n_queens(n: i32) -> Vec<Vec<String>> {
+    let mut board = vec![vec!['.'; n as usize]; n as usize];
+    let mut solution = vec![];
+    schedule_queens(&mut board, &mut solution, n as usize, 0);
+    solution
+}
+
+fn schedule_queens(board: &mut Vec<Vec<char>>, solution: &mut Vec<Vec<String>>, len: usize, row: usize) {
+    for col in 0..len {
+        if !collision(&board, len, row, col) {
+            board[row][col] = 'Q';
+            if row == len - 1 {
+                solution.push(board.iter().map(|vec| vec.iter().collect()).collect());
+            } else {
+                schedule_queens(board, solution, len, row+1);
+            }
+            board[row][col] = '.';
+        }
+    }
+}
+
+#[inline(always)]
+fn collision(board: &Vec<Vec<char>>, len: usize, row: usize, col: usize) -> bool {
+    for i in 0..row {
+        if board[i][col] == 'Q' { return true }
+    }
+    let (mut i, mut j) = (row as i32 - 1, col as i32 - 1);
+    while i >= 0 && j >= 0 {
+        if board[i as usize][j as usize] == 'Q' { return true }
+        i -= 1; j -= 1;
+    }
+    let (mut i, mut j) = (row as i32 - 1, col as i32 + 1);
+    while i >= 0 && j < len as i32 {
+        if board[i as usize][j as usize] == 'Q' { return true}
+        i -= 1; j += 1;
+    }
+    false
+}
+
+fn main() {
+    let m = solve_n_queens(8);
+    println!("{:?}", m);
+}