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;
}
}