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542. 01 Matrix 👍

Approach 1: w/ boolean array

  • Time: $O(mn)$
  • Space: $O(mn)$
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class Solution {
 public:
  vector<vector<int>> updateMatrix(vector<vector<int>>& mat) {
    const int m = mat.size();
    const int n = mat[0].size();
    const vector<int> dirs{0, 1, 0, -1, 0};
    queue<pair<int, int>> q;
    vector<vector<bool>> seen(m, vector<bool>(n));

    for (int i = 0; i < m; ++i)
      for (int j = 0; j < n; ++j)
        if (mat[i][j] == 0) {
          q.emplace(i, j);
          seen[i][j] = true;
        }

    while (!q.empty()) {
      const auto [i, j] = q.front();
      q.pop();
      for (int k = 0; k < 4; ++k) {
        const int x = i + dirs[k];
        const int y = j + dirs[k + 1];
        if (x < 0 || x == m || y < 0 || y == n)
          continue;
        if (seen[x][y])
          continue;
        mat[x][y] = mat[i][j] + 1;
        q.emplace(x, y);
        seen[x][y] = true;
      }
    }

    return mat;
  }
};

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class Solution {
  public int[][] updateMatrix(int[][] mat) {
    final int m = mat.length;
    final int n = mat[0].length;
    final int[] dirs = new int[] {0, 1, 0, -1, 0};
    Queue<int[]> q = new ArrayDeque<>();
    boolean[][] seen = new boolean[m][n];

    for (int i = 0; i < m; ++i)
      for (int j = 0; j < n; ++j)
        if (mat[i][j] == 0) {
          q.offer(new int[] {i, j});
          seen[i][j] = true;
        }

    while (!q.isEmpty()) {
      final int i = q.peek()[0];
      final int j = q.poll()[1];
      for (int k = 0; k < 4; ++k) {
        final int x = i + dirs[k];
        final int y = j + dirs[k + 1];
        if (x < 0 || x == m || y < 0 || y == n)
          continue;
        if (seen[x][y])
          continue;
        mat[x][y] = mat[i][j] + 1;
        q.offer(new int[] {x, y});
        seen[x][y] = true;
      }
    }

    return mat;
  }
}

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class Solution:
  def updateMatrix(self, mat: List[List[int]]) -> List[List[int]]:
    m = len(mat)
    n = len(mat[0])
    dirs = [0, 1, 0, -1, 0]
    q = deque()
    seen = [[False] * n for _ in range(m)]

    for i in range(m):
      for j in range(n):
        if mat[i][j] == 0:
          q.append((i, j))
          seen[i][j] = True

    while q:
      i, j = q.popleft()
      for k in range(4):
        x = i + dirs[k]
        y = j + dirs[k + 1]
        if x < 0 or x == m or y < 0 or y == n:
          continue
        if seen[x][y]:
          continue
        mat[x][y] = mat[i][j] + 1
        q.append((x, y))
        seen[x][y] = True

    return mat

Approach 2: w/o boolean array

  • Time: $O(mn)$
  • Space: $O(mn)$
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class Solution {
 public:
  vector<vector<int>> updateMatrix(vector<vector<int>>& mat) {
    const int m = mat.size();
    const int n = mat[0].size();
    const vector<int> dirs{0, 1, 0, -1, 0};
    queue<pair<int, int>> q;

    for (int i = 0; i < m; ++i)
      for (int j = 0; j < n; ++j)
        if (mat[i][j] == 0)
          q.emplace(i, j);
        else
          mat[i][j] = INT_MAX;

    while (!q.empty()) {
      const auto [i, j] = q.front();
      q.pop();
      for (int k = 0; k < 4; ++k) {
        const int x = i + dirs[k];
        const int y = j + dirs[k + 1];
        if (x < 0 || x == m || y < 0 || y == n)
          continue;
        if (mat[x][y] <= mat[i][j] + 1)
          continue;
        q.emplace(x, y);
        mat[x][y] = mat[i][j] + 1;
      }
    }

    return mat;
  }
};

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class Solution {
  public int[][] updateMatrix(int[][] mat) {
    final int m = mat.length;
    final int n = mat[0].length;
    final int[] dirs = new int[] {0, 1, 0, -1, 0};
    Queue<int[]> q = new ArrayDeque<>();

    for (int i = 0; i < m; ++i)
      for (int j = 0; j < n; ++j)
        if (mat[i][j] == 0)
          q.offer(new int[] {i, j});
        else
          mat[i][j] = Integer.MAX_VALUE;

    while (!q.isEmpty()) {
      final int i = q.peek()[0];
      final int j = q.poll()[1];
      for (int k = 0; k < 4; ++k) {
        final int x = i + dirs[k];
        final int y = j + dirs[k + 1];
        if (x < 0 || x == m || y < 0 || y == n)
          continue;
        if (mat[x][y] <= mat[i][j] + 1)
          continue;
        q.offer(new int[] {x, y});
        mat[x][y] = mat[i][j] + 1;
      }
    }

    return mat;
  }
}

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class Solution:
  def updateMatrix(self, mat: List[List[int]]) -> List[List[int]]:
    m = len(mat)
    n = len(mat[0])
    dirs = [0, 1, 0, -1, 0]
    q = deque()

    for i in range(m):
      for j in range(n):
        if mat[i][j] == 0:
          q.append((i, j))
        else:
          mat[i][j] = math.inf

    while q:
      i, j = q.popleft()
      for k in range(4):
        x = i + dirs[k]
        y = j + dirs[k + 1]
        if x < 0 or x == m or y < 0 or y == n:
          continue
        if mat[x][y] <= mat[i][j] + 1:
          continue
        q.append((x, y))
        mat[x][y] = mat[i][j] + 1

    return mat
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