01 Matrix LeetCode solution in C++

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class Solution {
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 public:
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  vector<vector<int>> updateMatrix(vector<vector<int>> &mat) {
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    queue<pair<int, int>> q;
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    for (int i = 0; i < mat.size(); i++) {
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      for (int j = 0; j < mat[0].size(); j++) {
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        if (mat[i][j] == 0)
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          q.push({i, j});
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        else
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          mat[i][j] = -1;
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      }
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    }
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    int length = 0;
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    int dirx[] = {1, -1, 0, 0};
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    int diry[] = {0, 0, 1, -1};
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    while (!q.empty()) {
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      int size = q.size();
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      length++;
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      while (size--) {
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        auto f = q.front();
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        q.pop();
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        int s = f.first;
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        int end = f.second;
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        for (int i = 0; i < 4; i++) {
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          int r = s + dirx[i];
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          int c = end + diry[i];
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          if (r < 0 || c < 0 || r == mat.size() || c == mat[0].size() || mat[r][c] != -1) continue;
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          mat[r][c] = length;
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          q.push({r, c});
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        }
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      }
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    }
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    return mat;
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  }
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};