/// Your problem was okay. I've Just replaced the dfs by bfs. dfs encounter stack overflow so you got RTE.
class Solution {
public:
    void solve(vector<vector<char> >&board) {
        int m=board.size();
        if(m==0) return;
        int n=board[0].size();

        for(int i=0;i<m;i++){
            if(board[i][0]=='O') {
                bfs(board, i, 0);
            }

            if(board[i][n-1]=='O') {
                bfs(board, i, n-1);
            }
        }

        for(int j=1;j<n-1;j++) {
            if(board[0][j]=='O') {
                bfs(board, 0, j);
            }

            if(board[m-1][j]=='O') {
                bfs(board, m-1, j);
            }
        }
        modify(board);
    }

    void bfs(vector<vector<char> >& board, int i, int j) {
        if(board[i][j]=='P') return;
        int m=board.size();
        int n=board[0].size();
        queue<pair<int,int> >Q;
        Q.push(make_pair(i,j));
        board[i][j]='P';
        while(!Q.empty()){
            pair<int,int>P=Q.front();
            Q.pop();
            int x=P.first,y=P.second;
            if(x+1<m && board[x+1][y]=='O'){
                board[x+1][y]='P';
                Q.push(make_pair(x+1,y));
            }
            if(x-1>=0 && board[x-1][y]=='O'){
                board[x-1][y]='P';
                Q.push(make_pair(x-1,y));
            }
            if(y+1<n && board[x][y+1]=='O'){
                board[x][y+1]='P';
                Q.push(make_pair(x,y+1));
            }
            if(y-1>=0 && board[x][y-1]=='O'){
                board[x][y-1]='P';
                Q.push(make_pair(x,y-1));
            }
        }
    }

    void modify(vector<vector<char> >& board) {
        int m=board.size();
        int n=board[0].size();
        for(int i=0;i<m;i++) {
            for(int j=0;j<n;j++) {
                if(board[i][j]=='P') {
                    board[i][j]='O';
                } else if(board[i][j]=='O') {
                    board[i][j]='X';
                }
            }
        }
    }
};