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_0052_NQueensII.java
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_0052_NQueensII.java
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package com.diguage.algorithm.leetcode;
/**
* = 52. N-Queens II
*
* https://leetcode.com/problems/n-queens-ii/[N-Queens II - LeetCode]
*
* @author D瓜哥, https://www.diguage.com/
* @since 2020-02-05 16:25
*/
public class _0052_NQueensII {
/**
* Runtime: 1 ms, faster than 95.68% of Java online submissions for N-Queens II.
* Memory Usage: 36 MB, less than 8.70% of Java online submissions for N-Queens II.
*/
private int result = 0;
public int totalNQueens(int n) {
int[][] matrix = new int[n][n];
backtrack(matrix, 0, 0);
return result;
}
private void backtrack(int[][] matrix, int y, int step) {
if (step == matrix.length) {
result++;
return;
}
for (int xi = 0; xi < matrix[0].length; xi++) {
if (isValid(matrix, y, xi)) {
matrix[y][xi] = 1;
backtrack(matrix, y + 1, step + 1);
matrix[y][xi] = 0;
}
}
}
private boolean isValid(int[][] matrix, int y, int x) {
int len = matrix.length;
// 同列
for (int i = 0; i < y; i++) {
if (matrix[i][x] == 1) {
return false;
}
}
// 左上角
for (int i = 1; i < len && y - i >= 0 && x - i >= 0; i++) {
if (matrix[y - i][x - i] == 1) {
return false;
}
}
// 右上角:从右上角到最下角的对角线,他们 "行号 + 列号 = 常数"
int sum = x + y;
for (int yi = y - 1; yi >= 0 && 0 <= sum - yi && sum - yi < len; yi--) {
if (matrix[yi][sum - yi] == 1) {
return false;
}
}
return true;
}
public static void main(String[] args) {
_0052_NQueensII solution = new _0052_NQueensII();
int r1 = solution.totalNQueens(4);
System.out.println((r1 == 2) + " : " + r1);
}
}