Leetcode 130 Surrounded Regions

原題地址

https://leetcode.com/problems/surrounded-regions/

題目描述

Given a 2D board containing ‘X’ and ‘O’, capture all regions surrounded by ‘X’.
給出一個2D圖板，又字符’X’和’O’組成，捕捉被’X’包圍的區域。

A region is captured by flipping all ‘O’s into ‘X’s in that surrounded region.
將被’X’包圍的區域全部置爲’X’就是對這個區域進行“逮捕”。

For example,
例如：

X X X X
X O O X
X X O X
X O X X

After running your function, the board should be:
在逮捕之後，應該變成：

X X X X
X X X X
X X X X
X O X X

解題思路

廣度優先遍歷

1. 最外層如果有O的話一定不會被包圍
2. 如果一個O不會被包圍，那他的周圍（上下左右）的O也不會被包圍。

算法描述

1. 拷貝原數據爲copy
2. 將周圍一圈中的’O’入棧queue
3. 原始數據的中間部分（除去周圍一圈）全部置爲’X’
4. 當queue不爲空時，取出棧頂元素，並判定其上下左右的4個位置的原始數據（copy中存儲着原始數據）是否爲’O’，如果是則將其置爲’O’

注意：爲了防止重複訪問，需要右標誌位visited[x][y]來記錄一個點是否已訪問過。

空間複雜度O(n)，時間複雜度O(n)。

代碼 cpp

class MyPoint {
public:
    int x;
    int y;
    MyPoint(int x, int y) : x(x), y(y) {}
};

class Solution {
public:
    void solve(vector<vector<char> > &board) {
        int rowSize = board.size(), colSize = rowSize > 0 ? board[0].size() : 0;
        if (rowSize <= 2 || colSize <= 2) return;

        vector<vector<char> > copy = vector<vector<char> >(board.begin(), board.end());
        vector<vector<bool> > visited = vector<vector<bool> >(rowSize, vector<bool>(colSize, false));

        --rowSize;
        --colSize;
        queue<MyPoint *> q;
        if (copy[0][0] == 'O') {
            q.push(new MyPoint(0, 0));
            visited[0][0] = true;
        }
        if (copy[0][colSize] == 'O') {
            q.push(new MyPoint(0, colSize));
            visited[0][colSize] = true;
        }
        if (copy[rowSize][0] == 'O') {
            q.push(new MyPoint(rowSize, 0));
            visited[rowSize][0] = true;
        }
        if (copy[rowSize][colSize] == 'O') {
            q.push(new MyPoint(rowSize, colSize));
            visited[rowSize][colSize] = true;
        }
        for (int i = 1; i < colSize; ++i) {
            if (copy[0][i] == 'O')
                q.push(new MyPoint(0, i));
            if (copy[rowSize][i] == 'O')
                q.push(new MyPoint(rowSize, i));
        }
        for (int i = 1; i < rowSize; ++i) {
            if (copy[i][0] == 'O')
                q.push(new MyPoint(i, 0));
            if (copy[i][colSize] == 'O')
                q.push(new MyPoint(i, colSize));
        }

        for (int i = 1; i < rowSize; ++i)
            for (int j = 1; j < colSize; ++j)
                board[i][j] = 'X';

        MyPoint *tmp;
        int dalx[] = {-1, 0, 1, 0}, daly[] = {0, 1, 0, -1};
        int x, y;
        while (!q.empty()) {
            tmp = q.front();
            q.pop();

            for (int i = 0; i < 4; ++i) {
                x = tmp->x + dalx[i];
                y = tmp->y + daly[i];
                if (x < 1 || y < 1 || x > rowSize || y > colSize)
                    continue;
                if (copy[x][y] == 'O' && !visited[x][y]) {
                    board[x][y] = 'O';
                    visited[x][y] = true;
                    q.push(new MyPoint(x, y));
                }                   
            }
            delete(tmp);
        }
    }
};

完整代碼https://github.com/Orange1991/leetcode/blob/master/130/cpp/main.cpp

運行情況

Status : Accept
Time : 20ms

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2015/7/22