从左上角洪泛改了几格

从 (0,0) 出发，把连着的 1 全改成 2。运行下面这段程序：

本节的网格（四行四列，1 是陆地，0 是海）：1100 / 1001 / 0010 / 1000。

#include <algorithm>
#include <iostream>
#include <queue>
#include <string>
#include <utility>
#include <vector>
using namespace std;

vector<vector<int>> GRID_MAP = {{1, 1, 0, 0}, {1, 0, 0, 1}, {0, 0, 1, 0}, {1, 0, 0, 0}};
const vector<pair<int, int>> D4 = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
const vector<pair<int, int>> D8 = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}, {1, 1}, {1, -1}, {-1, 1}, {-1, -1}};

int flood(vector<vector<int>>& g, int si, int sj, int nw) {
    int m = g.size(), n = g[0].size();
    int old = g[si][sj];
    if (old == nw) return 0;
    vector<pair<int, int>> st = {{si, sj}};
    g[si][sj] = nw;
    int k = 1;
    while (!st.empty()) {
        auto [x, y] = st.back();
        st.pop_back();
        for (auto [dx, dy] : D4) {
            int a = x + dx, b = y + dy;
            if (a >= 0 && a < m && b >= 0 && b < n && g[a][b] == old) {
                g[a][b] = nw;
                st.push_back({a, b});
                k++;
            }
        }
    }
    return k;
}

int main() {
    cout << flood(GRID_MAP, 0, 0, 2) << endl;
}

（本题用 g++ -std=c++17 -O0 编译。）