8.18 [LeetCode 52] N-Queens II

[LeetCode 52] N-Queens II

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Question

link

 

Follow up for N-Queens problem.

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

 

Stats

Frequency	3
Difficulty	4
Adjusted Difficulty	2
Time to use	——–

Ratings/Color = 1(white) 2(lime) 3(yellow) 4/5(red)

Solution

I posted 2 solution by me. Second code is same as this guy’s code.

My code

Using global variable (similar to [LeetCode 51] N-Queens)

public class Solution {
    int res = 0; // res should be global, cause java func cannot return int value back.
    public int totalNQueens(int n) {
        //int res = 0;
        //int[] col4row = new int[n];
        helper(n, 0, new int[n]);
        return res;
    }
    
    private void helper(int n, int row, int[] col4row) {
        if(row == n) { // all positions are ok
            ++res;
            return; // do not forget to quit from this recursion here.
        }
        for(int i = 0; i < n; i++) {
            col4row[row] = i; // put every col to this row to test the correctness.
            if(check(row, col4row)) {
                helper(n, row+1, col4row);
            }
        }
    }
    
    private boolean check(int row, int[] col4row) {
        for(int i = 0; i < row; i++) {
            if(col4row[i] == col4row[row] ||
                Math.abs(col4row[i]-col4row[row]) == row-i)
                return false;
        }
        return true;
    }
}

Without using global variable,

public class Solution {
    public int totalNQueens(int n) {
        return solve(0, n, new int[n]);
    }

    private int solve(int row, int n, int[] col4row) {
        if (row == n) return 1;
        int ans = 0;
        for (int i = 0; i < n; i++) {
            boolean conflict = false;
            for (int j = 0; j < row; j++) // check first, if not ok then go to next col, if ok let col4row[row] = i;
                if (i == col4row[j] || row - j == Math.abs(i - col4row[j]))
                    conflict = true;
            if (conflict) continue;
            col4row[row] = i;
            ans += solve(row + 1, n, col4row); // this row is finished, go to next row
        }
        return ans;
    }
}