314. Binary Tree Vertical Order Traversal

題目：

Given a binary tree, return the vertical order traversal of its nodes' values. (ie, from top to bottom, column by column).

If two nodes are in the same row and column, the order should be from left to right.

Examples:

Given binary tree 

[3,9,20,null,null,15,7]

,

3
   / \
  9  20
    /  \
   15   7
      

return its vertical order traversal as:

[
  [9],
  [3,15],
  [20],
  [7]
]
      

Given binary tree 

[3,9,20,4,5,2,7]

,

_3_
   /   \
  9    20
 / \   / \
4   5 2   7
      

return its vertical order traversal as:

[
  [4],
  [9],
  [3,5,2],
  [20],
  [7]

]


       
public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }
    private static void helper(TreeNode root, HashMap<Integer, ArrayList<Integer>> map, int ind){
        if(root==null) return;
        if(!map.containsKey(ind)){
            ArrayList<Integer> temp = new ArrayList<>();
            temp.add(root.val);
            map.put(ind,temp);
        }
        else{
            map.get(ind).add(root.val);
        }
        helper(root.left,map,ind-1);
        helper(root.right,map,ind+1);
    }

    public static List<List<Integer>> verticalOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if(root==null) return res;
        HashMap<Integer, ArrayList<Integer>> map = new HashMap<Integer, ArrayList<Integer>>();
        helper(root,map,0);
        //map to list
        int mn = Integer.MAX_VALUE, mx = Integer.MIN_VALUE;
        for(int i:map.keySet()){
            mn = Math.min(mn, i);
            mx = Math.max(mx, i);
        }
        for(int i=mn; i<=mx; i++)
            res.add(map.get(i));
        return res;
    }