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class Solution {
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 public:
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  vector<vector<int>> spiralMatrix(int n, int m, ListNode *head) {
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    // Create a matrix of n x m with values filled with -1.
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    vector<vector<int>> spiral(n, vector<int>(m, -1));
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    int i = 0, j = 0;
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    // Traverse the matrix in spiral form, and update with the values present in
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    // the head list. If head reacher NULL pointer break out from the loop, and
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    // return the spiral matrix.
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    while (head != NULL) {
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      if (j < m) {
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        while (head != NULL && j < m && spiral[i][j] == -1) {
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          spiral[i][j] = head->val;
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          head = head->next;
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          j++;
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        }
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        if (head == NULL) break;
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        i++;
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        j--;
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      }
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      if (i < n) {
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        while (head != NULL && i < n && spiral[i][j] == -1) {
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          spiral[i][j] = head->val;
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          head = head->next;
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          i++;
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        }
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        i--;
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        j--;
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      }
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      if (j >= 0) {
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        while (head != NULL && j >= 0 && spiral[i][j] == -1) {
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          spiral[i][j] = head->val;
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          head = head->next;
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          j--;
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        }
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        j++;
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        i--;
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      }
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      if (i >= 0) {
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        while (head != NULL && i >= 0 && spiral[i][j] == -1) {
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          spiral[i][j] = head->val;
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          head = head->next;
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          i--;
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        }
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        i++;
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        j++;
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      }
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      n--;
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      m++;
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    }
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    // Rest values are itself -1.
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    return spiral;
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  }
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};