SurroundedRegions

Change-Id: I5e4b11be68f210ac7380d8853abe968e24b997ab

Author: applewjg

Diff for: SurroundedRegions.java

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+/*
+ Author:     Andy, [email protected]
+ Date:       Jan 20, 2015
+ Problem:    Surrounded Regions
+ Difficulty: Easy
+ Source:     http://leetcode.com/onlinejudge#question_130
+ Notes:
+ Given a 2D board containing 'X' and 'O', capture all regions surrounded by 'X'.
+ A region is captured by flipping all 'O's into 'X's in that surrounded region .
+ For example,
+ X X X X
+ X O O X
+ X X O X
+ X O X X
+ After running your function, the board should be:
+ X X X X
+ X X X X
+ X X X X
+ X O X X
+
+ Solution: Traverse from the boarder to the inside and mark all the 'O's that are not surrounded by 'X' as 'V' (visited).
+           1. BFS (queue).
+ */
+public class Solution {
+    public void solve(char[][] board) {
+        if (board.length == 0 || board[0].length == 0) return;
+        int M = board.length, N = board[0].length;
+        for (int i = 0; i < M; ++i) {
+            for (int j = 0; j < N; ++j) {
+                if (i == 0 || j == 0 || i == M - 1 || j == N -1) {
+                    bfs(board, i, j);
+                }
+            }
+        }
+        for (int i = 0; i < M; ++i)
+            for (int j = 0; j < N; ++j)
+                board[i][j] = (board[i][j] == 'V') ? 'O' : 'X';
+    }
+    public void bfs(char[][] board, int row, int col) {
+        if (board[row][col] != 'O') return;
+        int M = board.length, N = board[0].length;
+        Queue<Integer> q = new LinkedList<Integer>();
+        q.offer(row); q.offer(col);
+        while (q.isEmpty() == false) {
+            int i = q.poll(); int j = q.poll();
+            if (i < 0 || i == M || j < 0 || j == N) continue;
+            if (board[i][j] != 'O') continue;// important to recheck!
+            board[i][j] = 'V';
+            q.offer(i-1); q.offer(j);
+            q.offer(i+1); q.offer(j);
+            q.offer(i); q.offer(j-1);
+            q.offer(i); q.offer(j+1);
+        }
+    }
+}