N-Queens II
https://leetcode.com/problems/n-queens-ii/
Follow up for N-Queens problem.
Now, instead outputting board configurations, return the total number of distinct solutions.
解题思路：
这题和N-Queens相比，有啥简单的方法？没看出，无非就是算出所有解后返回结果的size。当然可能也可以维护一个值，有结果就递增，最后返回他就可以了。
public class Solution { public int totalNQueens(int n) { List<String[]> result = new ArrayList<String[]>(); if(n < 1) { return 0; } String[] current = new String[n]; for(int i = 0; i < n; i++) { StringBuffer bf = new StringBuffer(); for(int j = 0; j < n; j++) { bf.append('.'); } current[i] = bf.toString(); } int[] columnInRow = new int[n]; for(int i = 0; i < n; i++) { columnInRow[i] = -1; } dfs(n, result, current, columnInRow, 0); return result.size(); } public void dfs(int n, List<String[]> result, String[] current, int[] columnInRow, int row) { if(row == n) { String[] temp = Arrays.copyOf(current, current.length); result.add(temp); return; } for(int i = 0; i < n; i++) { if(checkValid(columnInRow, row, i)) { columnInRow[row] = i; String temp = current[row]; current[row] = current[row].substring(0, i) + "Q" + current[row].substring(i + 1); dfs(n, result, current, columnInRow, row + 1); current[row] = temp; columnInRow[row] = -1; } } } public boolean checkValid(int[] columnInRow, int row, int column) { int temp = row - 1, i = 1; while(temp >= 0) { if(columnInRow[temp] == column) { return false; } if(column - i >= 0 && columnInRow[temp] == column - i) { return false; } if(column + i < columnInRow.length && columnInRow[temp] == column + i) { return false; } i++; temp--; } temp = row + 1; i = 1; while(temp < columnInRow.length) { if(columnInRow[temp] == column) { return false; } if(column - i >= 0 && columnInRow[temp] == column - i) { return false; } if(column + i < columnInRow.length && columnInRow[temp] == column + i) { return false; } i++; temp++; } return true; } }
又优化了一下省去了构造string的过程，并直接返回这个size。
public class Solution { public int totalNQueens(int n) { if(n < 1) { return 0; } int[] columnInRow = new int[n]; for(int i = 0; i < n; i++) { columnInRow[i] = -1; } int result = 0; result = dfs(n, columnInRow, 0, result); return result; } public int dfs(int n, int[] columnInRow, int row, int result) { if(row == n) { result++; return result; } for(int i = 0; i < n; i++) { if(checkValid(columnInRow, row, i)) { columnInRow[row] = i; result = dfs(n, columnInRow, row + 1, result); columnInRow[row] = -1; } } return result; } public boolean checkValid(int[] columnInRow, int row, int column) { int temp = row - 1, i = 1; while(temp >= 0) { if(columnInRow[temp] == column) { return false; } if(column - i >= 0 && columnInRow[temp] == column - i) { return false; } if(column + i < columnInRow.length && columnInRow[temp] == column + i) { return false; } i++; temp--; } temp = row + 1; i = 1; while(temp < columnInRow.length) { if(columnInRow[temp] == column) { return false; } if(column - i >= 0 && columnInRow[temp] == column - i) { return false; } if(column + i < columnInRow.length && columnInRow[temp] == column + i) { return false; } i++; temp++; } return true; } }