# 1424. Diagonal Traverse II
###### tags: `Leetcode` `Medium` `FaceBook` `Diagonal`
Link: https://leetcode.com/problems/diagonal-traverse-ii/
## 思路
利用同一条对角线的所有元素的row+col都相等的性质
构建hashmap 存下所有元素
但是由于遍历是从上往下遍历的，所以装进答案的时候需要反着装，因为对角线是从下往上的
## Code

class Solution {
    public int[] findDiagonalOrder(List<List<Integer>> nums) {
        Map<Integer, List<Integer>> map = new HashMap<>();
        int min = Integer.MAX_VALUE;
        int max = Integer.MIN_VALUE;
        int count = 0;
        for(int i = 0;i < nums.size();i++){
            List<Integer> curr = nums.get(i);
            int size = curr.size();
            for(int j = 0; j < size;j++){
                min = Math.min(min, i+j);
                max = Math.max(max, i+j);
                count++;
                if(!map.containsKey(i+j)){
                    map.put((i+j), new ArrayList<Integer>());
                }
                map.get(i+j).add(curr.get(j));
            }
        }
        int[] ans = new int[count];
        int idx = 0;
        for(int i = min;i <= max;i++){
            List<Integer> curr = map.get(i);
            int size = curr.size();
            for(int j = size-1;j >= 0;j--){
                ans[idx++] = curr.get(j);
            }
        }
        return ans;
    }
}