LeetCode 145. 二叉树的后序遍历

Posted 2021-10-05 Blocking The Sky

给定一个二叉树，返回它的 后序 遍历。

1、递归实现

class Solution {
public:
    vector<int> result;
    void PostOrder(TreeNode* root){
        if(root){
            PostOrder(root->left);
            PostOrder(root->right);
            result.push_back(root->val);
        }
    }
    vector<int> postorderTraversal(TreeNode* root) {
        PostOrder(root);
        return result;
    }
};

2、用栈非递归实现

class Solution {
public:
    vector<int> result;
    vector<int> postorderTraversal(TreeNode* root) {
        stack<TreeNode*> s;
        TreeNode *p;
        p=NULL;
        while(root||!s.empty()){
            if(root){
                s.push(root);
                root=root->left;
            }
            else{
                root=s.top();
                if(root->right&&root->right!=p){//判断是否已经访问过
                    root=root->right;
                }
                else{
                    s.pop();
                    result.push_back(root->val);
                    p=root;
                    root=NULL;//该子树访问结束
                }
            }
        }
        return result;
    }
};