All Elements in Two Binary Search Trees

给两个bst, 返回一个sorted list. 我直接用merge list做的, 反正复杂度都一样.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> getAllElements(TreeNode root1, TreeNode root2) {
        List<Integer> l1 = new ArrayList<>();
        List<Integer> l2 = new ArrayList<>();
        get(root1, l1);
        get(root2, l2);
        return merge(l1, l2);
    }
    
    private void get(TreeNode t, List<Integer> l) {
        if(t == null)
            return;
        get(t.left, l);
        l.add(t.val);
        get(t.right, l);
    }
    
    private List<Integer> merge(List<Integer> l1, List<Integer> l2) {
        int i = 0;
        int j = 0;
        List<Integer> res = new ArrayList<>();
        while(i < l1.size() && j < l2.size()) {
            if(l1.get(i) <= l2.get(j)) {
                res.add(l1.get(i++));
            }
            else {
                res.add(l2.get(j++));
            }
        }
        while(i < l1.size()) {
            res.add(l1.get(i++));
        }
        while(j < l2.size()) {
            res.add(l2.get(j++));
        }
        return res;
    }
}