Follow-up question for NQueens here: http://jznest.wordpress.com/2014/05/10/n-queens/
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public class NQueensII { | |
public int totalNQueens(int n) { | |
int num = 0; | |
int[][] boardStatus = new int[n][n]; | |
// use Integer object and void is a bad move. It seems the reference is not working. | |
// we probably need to wrap it in another object? | |
// in fact Integer is not necessary. We only increase the counter in the base case. | |
num = solveNQueens(num, boardStatus, 0, n); | |
return num; | |
} | |
private int solveNQueens(Integer num, int[][] boardStatus, int level, int n) { | |
if (level == n) return ++num; | |
for (int i = 0; i < n; i++) { | |
if (boardStatus[level][i] == 0) { | |
updateBoardStatus(level, i, boardStatus, n, false); // as well | |
num = solveNQueens(num, boardStatus, level + 1, n); | |
// undo | |
updateBoardStatus(level, i, boardStatus, n, true); | |
} | |
} | |
return num; | |
} | |
private void updateBoardStatus(int level, int i, int[][] boardStatus, int n, boolean isUndo) { | |
int change = isUndo ? -1 : 1; | |
int j = 0; | |
while (j < n) { | |
boardStatus[level][j] += change; | |
boardStatus[j++][i] += change; | |
} | |
boardStatus[level][i] -= change; // we updated twice to this cell in the loop above | |
// update the cross | |
int k = level; | |
int m = i; | |
while (--k >= 0 && --m >= 0) boardStatus[k][m] += change; | |
k = level; | |
m = i; | |
while (++k < n && ++m < n) boardStatus[k][m] += change; | |
k = level; | |
m = i; | |
while (++k < n && --m >= 0) boardStatus[k][m] += change; | |
k = level; | |
m = i; | |
while (--k >= 0 && ++m < n) boardStatus[k][m] += change; | |
} | |
} |
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