832. Flipping an Image

Given a binary matrix A, we want to flip the image horizontally, then invert it, and return the resulting image.

To flip an image horizontally means that each row of the image is reversed. For example, flipping [1, 1, 0] horizontally results in [0, 1, 1].

To invert an image means that each 0 is replaced by 1, and each 1 is replaced by 0. For example, inverting [0, 1, 1] results in [1, 0, 0].

Example 1:

Input: [[1,1,0],[1,0,1],[0,0,0]]
Output: [[1,0,0],[0,1,0],[1,1,1]]
Explanation: First reverse each row: [[0,1,1],[1,0,1],[0,0,0]].
Then, invert the image: [[1,0,0],[0,1,0],[1,1,1]]

Example 2:

Input: [[1,1,0,0],[1,0,0,1],[0,1,1,1],[1,0,1,0]]
Output: [[1,1,0,0],[0,1,1,0],[0,0,0,1],[1,0,1,0]]
Explanation: First reverse each row: [[0,0,1,1],[1,0,0,1],[1,1,1,0],[0,1,0,1]].
Then invert the image: [[1,1,0,0],[0,1,1,0],[0,0,0,1],[1,0,1,0]]

解题要点：

先设置两个指针，一个指向头，另一个指向尾，交换头尾数（in-place）以此完成flip。交换后invert数字，最后返回处理过的原数组。

class Solution {
    public int[][] flipAndInvertImage(int[][] A) {
        int n = A.length;
        for(int k = 0; k < n; k++){
            int i = 0, j = A.length - 1;
            while(i < j){
                flip(A[k], i, j);
                A[k][i] = 1 - A[k][i];
                A[k][j] = 1 - A[k][j];
                i++;
                j--;
            }
            if (i == j) A[k][i] = 1 - A[k][j];
        }              
        return A;
    }
    
    public void flip(int[] a, int i, int j){
        int temp = a[i];
        a[i] = a[j];
        a[j] = temp; 
    }
}