中序遍历 左根右,递归
class Solution {
Node head, last;
public Node treeToDoublyList(Node root) {
if (root == null) {
return null;
}
dfs(root);
head.left = last;
last.right = head;
return head;
}
void dfs(Node node) {
if (node == null) {
return;
}
dfs(node.left);
if (last != null) {
last.right = node;
} else {
head = node;
}
node.left = last;
last = node;
dfs(node.right);
}
}