二叉树前序中序后序遍历的非递归形式

前序遍历

/*
 * 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 preorderTraversal(TreeNode* root) 
    {
        vector ret;
        stack stk;
        if (!root) return ret;
        while (root || stk.size())
        {
            if (root)
            {
                stk.push(root);
                ret.push_back(root->val);
                root = root->left;
            }
            else
            {
                root = stk.top();
                stk.pop();
                root = root->right;
            }
        }
        return ret;    
    }
};

中序遍历

/*
 * 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 inorderTraversal(TreeNode* root) 
    {
        vector ret;
        stack stk;
        if (!root) return ret;
        while (root || stk.size())
        {
            if (root)
            {
                stk.push(root);
                root = root->left;
            }
            else
            {
                root = stk.top();
                stk.pop();
                ret.push_back(root->val);
                root = root->right;
            }
        }
        return ret;
    }
};

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 postorderTraversal(TreeNode* root) 
    {
        vector ret;
        stack stk;
        if (!root) return ret;
        TreeNode *last = nullptr;
        while (root || stk.size())
        {
            if (root)
            {
                stk.push(root);
                root = root->left;
            }
            else
            {
                TreeNode *now = stk.top();
                if (now->right && last != now->right)
                    root = now->right;
                else
                {
                    ret.push_back(now->val);
                    stk.pop();
                    last = now;
                }
            }
        }
        return ret;
    }
};