Emm BFS开了O2优化居然过了。不知道编译器施了什么魔法

#include <iostream>
#include <queue>
#include <algorithm>
using namespace std;

int dp[105][105], r, c, ans = -1, dx[4] = { 1, -1, 0, 0 }, dy[4] = { 0, 0, 1, -1 };
bool v[105][105];
long long g[105][105];

inline bool legal(const int x, const int y) {
    return x >= 0 && x < r && y >= 0 && y < c;
}

struct St {
    int x, y;
};

int main() {
    ios::sync_with_stdio(false);
    cin >> r >> c;
    for (int i = 0; i < r; ++i) {
        for (int j = 0; j < c; ++j) {
            dp[i][j] = 1;
        }
    }
    for (int i = 0; i < r; ++i)
        for (int j = 0; j < c; ++j) {
            cin >> g[i][j];
        }

    queue<St> q;
    for (int i = 0; i < r; ++i) {
        for (int j = 0; j < c; ++j) {
            q.push({ i, j });
        }
    }
    while (!q.empty()) {
        St f = q.front();
        q.pop();
        ans = max(ans, dp[f.x][f.y]);
        v[f.x][f.y] = false;
        for (int i = 0; i < 4; ++i) {
            int x = f.x + dx[i], y = f.y + dy[i];
            if (legal(x, y) && g[x][y] < g[f.x][f.y] && dp[x][y] < dp[f.x][f.y] + 1) {
                dp[x][y] = dp[f.x][f.y] + 1;
                if (!v[x][y]) {
                    v[x][y] = true;
                    q.push({ x, y });
                }
            }
        }
    }
    cout << ans;
}