PROBLEM DESCRIPTION
Give a N x N square matrix, return an array of its anti-diagonals. Look at the example for more details.
Input:
1
2
3
1 2 3
4 5 6
7 8 9
Output:
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2
3
4
5
6
7
[
[1],
[2, 4],
[3, 5, 7],
[6, 8],
[9]
]
SOLUTION
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public class Solution {
public ArrayList<ArrayList<Integer>> diagonal(ArrayList<ArrayList<Integer>> A) {
int n = A.size();
ArrayList<ArrayList<Integer>> diagonals = new ArrayList<>();
// Diagonals starting from row 0
for(int i = 0; i < n; i++){
int r = 0; // Start from the first row
int c = i; // The current diagonal will start from column at index i
ArrayList<Integer> diag = new ArrayList<>();
// The next position will be A[r++][c--]
while(c >= 0 && r < n){
diag.add(A.get(r).get(c));
r++;
c--;
}
diagonals.add(diag);
}
// Diagonals starting from last column (except topmost)
// The diagonal starting from top right corner was already covered in the previous loop
for(int i = 1; i < n; i++){
int r = i; // The diagonal will start from row at index i
int c = n - 1; // The diagonal will start from the last column
ArrayList<Integer> diag = new ArrayList<>();
// // The next position will be A[r++][c--]
while(c >= 0 && r < n){
diag.add(A.get(r).get(c));
r++;
c--;
}
diagonals.add(diag);
}
return diagonals;
}
}