Spiral Matrix Java

最新推荐文章于 2024-01-15 18:28:38 发布

Spencer_Lin

最新推荐文章于 2024-01-15 18:28:38 发布

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分类专栏： LeetCode String+Array 文章标签： leetcode matrix

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本文详细阐述了如何使用4个独立循环遍历并获取矩阵元素的螺旋顺序，包括初始化边界指针、按层遍历矩阵等关键步骤，并通过实例代码进行演示。

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

For example,
Given the following matrix:

[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]

You should return [1,2,3,6,9,8,7,4,5].

Have you been asked this question in an interview?

Check the coding in detail below:

Key to solve: 4 individual loops,
Time: O(m*n)
1. Set up the 4 border pointers
2. read each elements inside matrix exactly once
by order of layer by layer ( from outside => inside)
1st loop: left-top -> right-top =>
2nd loop: right-top -> right-bottom =>
3rd loop: bottom-right-> bottom-left =>
4th loop: left-bottom -> left-up

public class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        ArrayList<Integer> out = new ArrayList<Integer>();
        //check for corner case
        if(matrix==null || matrix.length==0) return out;
        //initial border pointers
        int minRow = 0;
        int minCol = 0;
        int maxRow = matrix.length - 1;
        int maxCol = matrix[0].length - 1;
        while (minRow <= maxRow && minCol <= maxCol) {
            //left-top    ->right-top
            for (int i = minCol; i <= maxCol; i++) {
                out.add(matrix[minRow][i]);
            }
            //right-top    ->  right-bottom
            for (int i = minRow + 1; i <= maxRow; i++) {
                out.add(matrix[i][maxCol]);
            }
            // right-bottom ->  left-bottom
            if (minRow != maxRow) {
                for (int i = maxCol - 1; i >= minCol; i--) {
                    out.add(matrix[maxRow][i]);
                }
            }
            //left-bottom ->  left-up
            if (minCol != maxCol) {
                for (int i = maxRow - 1; i > minRow; i--) {
                    out.add(matrix[i][minCol]);
                }
            }
            //move border pointers of outside layer into inside
            minRow++;
            minCol++;
            maxRow--;
            maxCol--;
        }
        return out;
    }
}