11/8 <matrix> LC 48 54 59

48. Rotate Image

先按对角线对称图形，再水平对折。

class Solution {
    public void rotate(int[][] matrix) {
        //1.transpose
        for(int i = 0; i < matrix.length; i++){
            for(int j = 0; j < i; j++){
                int temp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = temp;
            }
        }
        //flip the matrix horizontally
        for(int i = 0; i < matrix.length; i++){
            for(int j = 0; j < matrix[0].length / 2; j++){
                int temp = matrix[i][j];
                matrix[i][j] = matrix[i][matrix[0].length - 1 -j];
                matrix[i][matrix[0].length - 1 -j] = temp;
            }
        }
    }
}

54. Spiral Matrix

从右到左，从下到上的时候，注意保证不重复。

class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        List<Integer> res = new ArrayList<Integer>();
        if(matrix == null || matrix.length == 0 || matrix[0].length == 0)
            return res;
        int rowBegin = 0, rowEnd = matrix.length - 1, colBegin = 0, colEnd = matrix[0].length - 1;
        while(rowBegin <= rowEnd && colBegin <= colEnd){
            //to right
            for(int j = colBegin; j <= colEnd; j++)  res.add(matrix[rowBegin][j]);
            rowBegin++;

            for(int i = rowBegin; i <= rowEnd; i++)  res.add(matrix[i][colEnd]);
            colEnd--;

            for(int j = colEnd; j >= colBegin && rowBegin <= rowEnd; j--)  res.add(matrix[rowEnd][j]);
            rowEnd--;

            for(int i = rowEnd; i >= rowBegin && colBegin <= colEnd; i--) res.add(matrix[i][colBegin]);
            colBegin++;
        }
        return res;
    }
}

59. Spiral Matrix II

规则的放入数字，不需要第三四步判断是否超出边界。

class Solution {
    public int[][] generateMatrix(int n) {
        if(n == 0)
            return null;
        int[][] matrix = new int[n][n];
        int rowBegin = 0, rowEnd = n - 1, colBegin = 0, colEnd = n - 1;
        int count = 0;
        while(rowBegin <= rowEnd && colBegin <= colEnd){
            //right
            for(int j = colBegin; j <= colEnd; j++)
                matrix[rowBegin][j] = ++count;
            rowBegin++;

            //down
            for(int i = rowBegin; i <= rowEnd; i++)
                matrix[i][colEnd] = ++count;
            colEnd--;

            //left
            for(int j = colEnd; j >= colBegin; j--)
                matrix[rowEnd][j] = ++count;
            rowEnd--;

            //up
            for(int i = rowEnd; i >= rowBegin; i--)
                matrix[i][colBegin] = ++count;
            colBegin++;
        }
        return matrix;
    }
}