题解 | #Sudoku#

我也是dfs+回溯+剪枝，各位看看我判定局面的思路即可

// https://www.nowcoder.com/practice/78a1a4ebe8a34c93aac006c44f6bf8a1
#include <cstring>
#include <iostream>
using namespace std;

const int maxn = 5 + 9;

int a[maxn][maxn];

const int sx[] = {0, 1, 1, 1, 4, 4, 4, 7, 7, 7};
const int sy[] = {0, 1, 4, 7, 1, 4, 7, 1, 4, 7};

bool inside(int x, int y) { return 1 <= x && x <= 9 && 1 <= y && y <= 9; }

bool accept() {
    static int cnt[10][10][3];    // row, col, squared
    memset(cnt, 0, sizeof(cnt));  // 第 s 个九宫格
    for (int s = 1; s <= 9; ++s) {
        for (int i = 0; i < 3; ++i) {
            for (int j = 0; j < 3; ++j) {
                int x = sx[s] + i, y = sy[s] + j;
                if (a[x][y] == 0) continue;
                if (++cnt[a[x][y]][x][0] > 1) return false;
                if (++cnt[a[x][y]][y][1] > 1) return false;
                if (++cnt[a[x][y]][s][2] > 1) return false;
            }
        }
    }
    return true;
}

void dfs(int x, int y) {
    if (x == 9 && y > 9) {  // find ans
        for (int i = 1; i <= 9; ++i) {
            for (int j = 1; j <= 9; ++j) {
                cout << a[i][j] << ' ';
            }
            cout << endl;
        }
        exit(0);
    }
    if (!inside(x, y)) {
        dfs(x + 1, 1);
        return;
    }

    if (a[x][y] != 0) {
        dfs(x, y + 1);
        return;
    }

    // a[x][y] == 0, inside(x, y)
    for (int i = 1; i <= 9; ++i) {
        a[x][y] = i;
        if (accept()) dfs(x, y + 1);
        a[x][y] = 0;
    }
}

int main() {
    for (int i = 1; i <= 9; ++i) {
        for (int j = 1; j <= 9; ++j) {
            cin >> a[i][j];
        }
    }
    dfs(1, 1);
}