add 48

Author: andavid

README.md

@@ -25,6 +25,8 @@
 | 283  | [Move Zeroes][283]                          |
 | 1    | [Two Sum][001]                              |
 | 36   | [Valid Sudoku][036]                         |
+| 48   | [Rotate Image][048]                         |
+
 
 **其他**
 
@@ -49,3 +51,4 @@
 [283]: https://github.com/andavid/leetcode-java/blob/master/note/283/README.md
 [001]: https://github.com/andavid/leetcode-java/blob/master/note/001/README.md
 [036]: https://github.com/andavid/leetcode-java/blob/master/note/036/README.md
+[048]: https://github.com/andavid/leetcode-java/blob/master/note/048/README.md

note/048/README.md

@@ -0,0 +1,76 @@
+# [Rotate Image][title]
+
+## Description
+
+You are given an n x n 2D matrix representing an image.
+
+Rotate the image by 90 degrees (clockwise).
+
+**Note:**
+You have to rotate the image **in-place**, which means you have to modify the input 2D matrix directly. **DO NOT** allocate another 2D matrix and do the rotation.
+
+**Example 1:**
+
+```
+Given input matrix = 
+[
+  [1,2,3],
+  [4,5,6],
+  [7,8,9]
+],
+
+rotate the input matrix in-place such that it becomes:
+[
+  [7,4,1],
+  [8,5,2],
+  [9,6,3]
+]
+```
+
+**Example 2:**
+
+```
+Given input matrix =
+[
+  [ 5, 1, 9,11],
+  [ 2, 4, 8,10],
+  [13, 3, 6, 7],
+  [15,14,12,16]
+], 
+
+rotate the input matrix in-place such that it becomes:
+[
+  [15,13, 2, 5],
+  [14, 3, 4, 1],
+  [12, 6, 8, 9],
+  [16, 7,10,11]
+]
+```
+
+## 思路
+
+题意是给一个 n x n 的二维数组，将数组元素顺时针旋转 90 度。需要直接修改当前数组。
+
+思路是从外往内，一圈一圈的进行。其中每一圈的每个元素顺时针旋转 90 度，连续操作四次。注意边界，防止重复操作。
+
+## [完整代码][src]
+
+```java
+class Solution {
+  public void rotate(int[][] matrix) {
+    int n = matrix.length;
+    for (int i = 0; i < n / 2; i ++) {
+      for (int j = i; j < n - 1 - i; j ++) {
+        int temp = matrix[i][j];
+        matrix[i][j] = matrix[n - 1 -j][i];
+        matrix[n - 1 -j][i] = matrix[n - 1 -i][n - 1 -j];
+        matrix[n - 1 - i][n - 1 - j] = matrix[j][n - 1 -i];
+        matrix[j][n - 1 -i] = temp;
+      }
+    }
+  }
+}
+```
+
+[title]: https://leetcode.com/problems/rotate-image
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_048/Solution.java

src/com/andavid/leetcode/_048/Solution.java

@@ -0,0 +1,32 @@
+class Solution {
+  public void rotate(int[][] matrix) {
+    int n = matrix.length;
+    for (int i = 0; i < n / 2; i ++) {
+      for (int j = i; j < n - 1 - i; j ++) {
+        int temp = matrix[i][j];
+        matrix[i][j] = matrix[n - 1 -j][i];
+        matrix[n - 1 -j][i] = matrix[n - 1 -i][n - 1 -j];
+        matrix[n - 1 - i][n - 1 - j] = matrix[j][n - 1 -i];
+        matrix[j][n - 1 -i] = temp;
+      }
+    }
+  }
+
+  public static void main(String[] args) {
+    Solution solution = new Solution();
+    int[][] matrix = {
+      {1, 2, 3},
+      {4, 5, 6},
+      {7, 8, 9}
+    };
+
+    solution.rotate(matrix);
+
+    for (int i = 0; i < matrix.length; i++) {
+      for (int j = 0; j < matrix[0].length; j++) {
+        System.out.print(j == 0 ? matrix[i][j] : "," + matrix[i][j]);
+      }
+      System.out.println();
+    }
+  }
+}