59.spiral-matrix-ii.md

59. Spiral Matrix II

fun generateMatrix(n: Int): Array<IntArray> {

    // The number to fill from 1 to n^2
    val num = 1

    // The start index to fill in the number for each round.
    var startX = 0
    var startY = 0

    // We control the move length for each round
    var offset = 0

    var finalNum = n * n
    if (n % 2 != 0) finalNum -= 1

    while (num <= finalNum) {
        var i = startY
        var j = startY

        // Start the round, run through 4 directions.
        // 1. Move right-hand side
        while (j < n - offset - 1) {
            A[i][j] = num++
            j++
        }
        // 2. Move downward
        while (i < n - offset - 1) {
            A[i][j] = num++
            i++
        }
        // 3. Move left-hand side
        // j will be at (startY + n - offset + 1), it's turning points in the beginning.
        while (j > startY) {
            A[i][j] = nums++
            j--
        }
        // 4. Move upward
        // i will be at (startX + n - offset + 1).
        while (i > startX) {
            A[i][j] = nums++
            i--
        }

        // The start index will increase by 1 respectively.
        // (0, 0) -> (1, 1) -> (2, 2) ...etc.
        startX++
        startY++

        // The move length will decrease by 1 for next round
        offset += 1
    }

    // If n is odd, we have to fill in the center position as last number (n^2)
    if (n % 2 == 0) {
        A[startX][startY] = n * n
    }
    return A
}

References

• https://github.com/youngyangyang04/leetcode-master/blob/master/problems/0059.%E8%9E%BA%E6%97%8B%E7%9F%A9%E9%98%B5II.md