_1424.java

packagecom.fishercoder.solutions.secondthousand;

importjava.util.*;

publicclass_1424 {
publicstaticclassSolution1 {
/*
 * One key note:
 * For all elements on the same diagonal, the sums of their row index and column index are equal.
 * This is widely applicable to all matrix problems.
 */
publicint[] findDiagonalOrder(List<List<Integer>> nums) {
TreeMap<Integer, List<Integer>> map = newTreeMap<>();
for (inti = 0; i < nums.size(); i++) {
for (intj = 0; j < nums.get(i).size(); j++) {
intindex = i + j;
List<Integer> list = map.getOrDefault(index, newArrayList<>());
list.add(0, nums.get(i).get(j));
map.put(index, list);
 }
 }
List<Integer> list = newArrayList<>();
for (intindex : map.keySet()) {
list.addAll(map.get(index));
 }
returnlist.stream().mapToInt(Integer -> Integer).toArray();
 }
 }
}