题目：
题解：
class Solution {public List<Integer> postorderTraversal(TreeNode root) {List<Integer> res = new ArrayList<Integer>();if (root == null) {return res;}TreeNode p1 = root, p2 = null;while (p1 != null) {p2 = p1.left;if (p2 != null) {while (p2.right != null && p2.right != p1) {p2 = p2.right;}if (p2.right == null) {p2.right = p1;p1 = p1.left;continue;} else {p2.right = null;addPath(res, p1.left);}}p1 = p1.right;}addPath(res, root);return res;}public void addPath(List<Integer> res, TreeNode node) {int count = 0;while (node != null) {++count;res.add(node.val);node = node.right;}int left = res.size() - count, right = res.size() - 1;while (left < right) {int temp = res.get(left);res.set(left, res.get(right));res.set(right, temp);left++;right--;}}
}