数据结构||二叉树的三种遍历方式(非递归)

二叉树的三种遍历方式(非递归)

//都是leetcode上的二叉树练习题,利用栈先进后出的特性用来标记根节点，然后进行一系列出栈入栈操作最终得到结果.

1.先序遍历

class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> res;
        TreeNode* cur = root;
        stack<TreeNode*> st;
        while(cur||!st.empty())
        {
            if(cur)
            {
                res.push_back(cur->val);
                st.push(cur);
                cur = cur->left;
            }
            else
            {
                cur = st.top();
                st.pop();
                cur = cur->right;
            }
        }
        return res;
    }
};

2.中序遍历

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        TreeNode* cur = root;
        stack<TreeNode*> st;
        while(cur || !st.empty())
        {
            while(cur)
            {
                st.push(cur);
                cur = cur->left;
            }
            cur = st.top();
            res.push_back(cur->val);
            st.pop();
            cur = cur->right;
        }
        return res;
    }
};

3.后序遍历

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        
        vector<int> res;
        stack<TreeNode*> s;
        TreeNode* p = root;
        TreeNode* r = nullptr;

        while(!s.empty() || p){
            if(p){
                s.push(p);
                p = p->left;
            }else{
                p = s.top();
                if(p->right == nullptr || p->right == r){
                    res.push_back(p->val);
                    s.pop();
                    r = p;
                    p = nullptr;
                }else 
                    p = p->right;
            }
        }
        return res;
    }