二叉树的前序、中序、后序遍历非递归做法

我们知道递归实际上是一个压栈的操作，故要写成非递归，我们需要利用栈。

前序：

/**
 * 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> preorderTraversal(TreeNode* root) {
        vector<int>res;
        stack<TreeNode*>stk;
        auto p=root;
        while(p||stk.size()){
            while(p){
                res.push_back(p->val);
                stk.push(p);
                p=p->left;
            }
            if(stk.size()){
                p=stk.top();
                stk.pop();
                p=p->right;
            }
        }
        return res;
    }
};

中序：

/**
 * 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> inorderTraversal(TreeNode* root) {
        stack<TreeNode*>stk;
        vector<int>res;
        auto p=root;
        while(p||stk.size()){
            while(p){
                stk.push(p);
                p=p->left;
            }
            p=stk.top();
            stk.pop();
            res.push_back(p->val);
            p=p->right;
        }
        return res;
    }
};

后序：

/**
 * 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>res;
        stack<TreeNode*>stk;
        TreeNode* p=root,*q=NULL;
        while(p||stk.size()){
            if(p){
                stk.push(p);
                p=p->left;
            }else{
                p=stk.top();
                if(p->right&&p->right!=q)p=p->right;
                else{
                    p=stk.top();
                    stk.pop();
                    res.push_back(p->val);
                    q=p;
                    p=NULL;
                }
            }
        }
        return res;
    }
};