leetcode 538. 把二叉搜索树转换为累加树

 1     public TreeNode convertBST(TreeNode root) {
 2         if (root == null) return root;
 3         HashMap<TreeNode, Integer> map = new HashMap<>();
 4         dfs(root, root, map);
 5         dfs2(root, map);
 6         return root;
 7     }
 8 
 9     public void dfs2(TreeNode r, HashMap<TreeNode, Integer> map) {
10         if (r==null)return;
11         r.val+=map.get(r);
12         dfs2(r.left,map);
13         dfs2(r.right,map);
14     }
15 
16     public void dfs(TreeNode n, TreeNode r, HashMap<TreeNode, Integer> map) {
17         if (n == null) return;
18         map.put(n, add(r, n.val));
19         dfs(n.left, r, map);
20         dfs(n.right, r, map);
21     }
22 
23 
24     public int add(TreeNode cur, int val) {
25         if (cur == null) return 0;
26         int sum = 0;
27         if (cur.val > val) sum += cur.val;
28         sum += add(cur.left, val);
29         sum += add(cur.right, val);
30         return sum;
31     }