学步园

先是中序遍历

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode *root) {
        vector<int>v;
        stack<TreeNode*>s;
        TreeNode* p = root;
        while(!s.empty() || p != NULL){
            if(p != NULL){
                s.push(p);
                p = p->left;
            }else {
                p = s.top();
                s.pop();
                v.push_back(p->val);
                p = p->right;
            }
        } 
        return v;
    }
};

然后是前序遍历

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
         vector<int>v;
         if(root == NULL) return v;
         stack<TreeNode*>s;
         s.push(root);
         while(!s.empty()){
             TreeNode *tmp = s.top();
              s.pop();
             v.push_back(tmp->val);
             if(tmp->right) s.push(tmp->right);
             if(tmp->left) s.push(tmp->left);
         }
         return v;
    }
};

后序遍历

/**
 * Definition for binary tree
 * 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) {
          stack<TreeNode*>s;
          vector<int>ans;
          if(root == NULL) return ans;   
          TreeNode *p = root, *q;
          do{
             while(p){
                s.push(p);
                p = p->left;
             }
           q = NULL;
            while(!s.empty()){
                p = s.top();
                s.pop();
                if(p->right == q){
                   ans.push_back(p->val);
                   q = p;
                }else {
                    s.push(p);
                    p = p->right;
                    break;
                }
            }
          }while(!s.empty());
          return ans;
    }
};

or

vector<int> postorderTraversal(TreeNode *root) {
    vector<int> v;
    if (!root) return v;

    stack<TreeNode *> s;
    s.push(root);

    TreeNode *p = NULL;
    while(!s.empty()) {
        p = s.top();
        s.pop();
        v.insert(v.begin(), p->val);
        if (p->left) s.push(p->left);
        if (p->right) s.push(p->right);
    }

    return v;
}