LC 144. / 94. / 145. Binary Tree Preorder/ Inorder/ PostOrder Traversal

题目描述

144. Binary Tree Preorder Traversal

94. Binary Tree Inorder Traversal

145. Binary Tree Postorder Traversal

前序排列 :根-左-右

中序排列: 左-根-右

后序排列:左-右-根

参考答案

// PreOrder
/**
 * 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 {
private:
        vector<int> res;

public:
    vector<int> preorderTraversal(TreeNode* root) {
        foo(root);
        return res;
    }
    void foo(TreeNode* root){
        if(root == NULL)
            return;
        
        res.push_back(root->val);
        foo(root->left);
        foo(root->right);
    }
};

// InOrder
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        if(root == NULL){
            return vector<int>();
        }
        vector<int> result; // 创建结果
        stack<TreeNode *> s;// 创建堆栈
        TreeNode *p = root; // 临时让p为root
        // 寻找p最左边的node
        while(p!=NULL){
            s.push(p); // 从root开始，将左边的node推入stack 
            p = p->left;// 更新p为左node
        }
        // s 为全是左节点的stack
        // 对 s进行循环操作
        while(!s.empty()){
            // 将最最左边的，推入stack
            p = s.top();
            result.push_back(p->val);
            s.pop(); // 自己消失了
            // 然后观察这个的右边node
            if(p->right != NULL){
                p = p->right;
                while(p!=NULL){ //观察node的左边
                    s.push(p);
                    p = p->left;
                }
            }     
        }
        return result;
    }
};

// PostOrder

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        if(root == NULL){
            return vector<int>();
        }
        vector<int> result;
        stack<TreeNode *> s;
        
        s.push(root);
        while(!s.empty()){
            TreeNode *temp = s.top();
            result.push_back(temp->val);
            s.pop();
            if(temp->left!=NULL){
                s.push(temp->left);
            }
            if(temp->right!= NULL){
                s.push(temp->right);
            }
        }
        reverse(result.begin(), result.end());
        return result;
        
    }
};