​LeetCode刷题实战145：二叉树的后序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 * int val;
 * TreeNode *left;
 * TreeNode *right;
 * TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
  vector<int> postorderTraversal(TreeNode* root) {
    vector<int> result;
    set<TreeNode*> visited;//用于标记已经访问过的节点
    stack<TreeNode*> myStack;//辅助栈，用于保留现场
    while (root != NULL || !myStack.empty()) {
      if (root != NULL) {
        myStack.push(root);//栈先保护根节点的现场
        visited.insert(root);//标记根节点已访问
        if (root->right) {//右子树不为空，保护右子树现场
          myStack.push(root->right);
        }
        root = root->left;//现场转移至左子树
        continue;
      }
      root = myStack.top();//恢复现场
      myStack.pop();
      while (visited.find(root) != visited.end()) {//如果这个节点已经访问过(根节点)
        result.push_back(root->val);
        if (!myStack.empty()) {
          root = myStack.top();//再恢复现场
          myStack.pop();
        }
        else {
          root == NULL;
                    return result;
        }
      }
    }
    return result;
  }
};