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| class Solution {
public:
const int MOD = 1000000007;
int memo[1000][1024];
int m, n;
int colorTheGrid(int m, int n) {
this->m = m, this->n = n;
return dp(0, 0);
}
int dp(int c, int prevColMask) {
if (c == n) return 1;
if (memo[c][prevColMask]) return memo[c][prevColMask];
vector<int> neighbors;
dfs(0, prevColMask, 0, neighbors);
int ans = 0;
for (int nei : neighbors)
ans = (ans + dp(c + 1, nei)) % MOD;
return memo[c][prevColMask] = ans;
}
void dfs(int r, int prevColMask, int currColMask, vector<int>& ans) {
if (r == m) {
ans.emplace_back(currColMask);
return;
}
for (int i = 1; i <= 3; ++i)
if (getColor(prevColMask, r) != i && (r == 0 || getColor(currColMask, r - 1) != i))
dfs(r + 1, prevColMask, setColor(currColMask, r, i), ans);
}
int getColor(int mask, int pos) {
return (mask >> (2 * pos)) & 3;
}
int setColor(int mask, int pos, int color) {
return mask |= (color << (2 * pos));
}
};
|