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NQueensII.java

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/**

* Follow up for N-Queens problem.

*

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

*

*/

public class NQueensII {

public int solveNQueens(int n) {

if (n == 0)

return 0;

StringBuffer line = new StringBuffer();

for (int i = 0; i < n; i++) {

line.append('.');

}

StringBuffer[] sol = new StringBuffer[n];

for (int i = 0; i < n; i++) {

sol[i] = new StringBuffer(line.toString());

}

boolean[] cols = new boolean[n];

int[] row = new int[n];

int[] count = new int[1];

findSolutions(n, 0, sol, row, cols, count);

return count[0];

}

private void findSolutions(int n, int start, StringBuffer[] sol, int[] row,

boolean[] cols, int[] count) {

if (start == n) {

count[0]++;

} else {

for (int i = 0; i < n; i++) {

if (cols[i])

continue;

boolean ok = true;

for (int j = 0; j < start; j++) {

if (Math.abs(start - j) == Math.abs(i - row[j])) {

ok = false;

break;

}

}

if (ok) {

cols[i] = true;

sol[start].setCharAt(i, 'Q');

row[start] = i;

findSolutions(n, start + 1, sol, row, cols, count);

row[start] = 0;

sol[start].setCharAt(i, '.');

cols[i] = false;

}

}

}

}

}