Spiral Matrix

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

Example 1:

Input: [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ] Output: [1,2,3,6,9,8,7,4,5] Example 2:

Input: [ [1, 2, 3, 4], [5, 6, 7, 8], [9,10,11,12] ] Output: [1,2,3,4,8,12,11,10,9,5,6,7]

Solution:

• Use dir to indicate direction 0,1,2,3
• Use rLow, rHigh and cLow, cHigh to mark range

vector<int> spiralOrder(vector<vector<int>>& matrix) {
        if (matrix.empty() || matrix.front().empty()) return {};

        int dir = 0;
        int rLow = 0, rHigh = matrix.size()-1;
        int cLow = 0, cHigh = matrix.front().size()-1;
        vector<int> ans;
        while (rLow <= rHigh && cLow <= cHigh) {
            if (dir == 0) {
                // Right
                for (int i=cLow; i<=cHigh; i++) {
                    ans.push_back(matrix[rLow][i]);
                }
                rLow += 1;
            } else if (dir == 1) {
                // Down
                for (int i=rLow; i<=rHigh; i++) {
                    ans.push_back(matrix[i][cHigh]);
                }
                cHigh -= 1;
            } else if (dir == 2) {
                // Left
                for (int i=cHigh; i>=cLow; i--) {
                    ans.push_back(matrix[rHigh][i]);
                }
                rHigh -= 1;
            } else if (dir == 3) {
                // Up
                for (int i=rHigh; i>=rLow; i--) {
                    ans.push_back(matrix[i][cLow]);
                }
                cLow += 1;
            }
            dir = (dir + 1) % 4;
        }

        return ans;
    }

vector<int> spiralOrder(vector<vector<int>>& matrix) {
        // 1. We start from position (0, 0)
        // 2. Follow the order right, down, left, up
        // 3. We change the direction each time we meet the boarder or visited position.
        // Corner case
        if (matrix.empty() || matrix.front().empty()) return {};

        // Init currrent direction and four directions
        int dir = 0;
        vector<vector<int>> dirs = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};

        // Init current position
        int row = 0, col = 0;

        // Use range to mark the left and right bound, top and bottom bound
        int rowTop = 0, rowBottom = matrix.size()-1;
        int colLeft = 0, colRight = matrix.front().size()-1;

        vector<int> ans;
        while (rowTop <= rowBottom && colLeft <= colRight) {
            // Move towards one direction
            while (row >= rowTop && row <= rowBottom &&
                   col >= colLeft && col <= colRight) {
                ans.push_back(matrix[row][col]);
                row += dirs[dir][0];
                col += dirs[dir][1];   
            }

            // Shrink range based on direction
            if (dir == 0) {
                rowTop += 1;
            } else if (dir == 1) {
                colRight -= 1;
            } else if (dir == 2) {
                rowBottom -= 1;
            } else {
                colLeft += 1;
            }

            // Step back one position
            row -= dirs[dir][0];
            col -= dirs[dir][1];

            // Change direction
            dir = (dir + 1) % 4;

            // Step forward to next position
            row += dirs[dir][0];
            col += dirs[dir][1];

        }
        return ans;
    }