leetcode_94_二叉樹的中序遍歷

給定一個二叉樹，返回它的中序 遍歷。

示例:

輸入: [1,null,2,3]
   1
    \
     2
    /
   3

輸出: [1,3,2]

進階: 遞歸算法很簡單，你可以通過迭代算法完成嗎？

/**
 * 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> result;
    void LDR(TreeNode* root)
    {
        if(root==NULL) 
            return;
        LDR(root->left);
        result.push_back(root->val);
        LDR(root->right);
    }
    vector<int> inorderTraversal(TreeNode* root) {
        LDR(root);
        return result;
    }
    
};

遞歸方法

/**
 * 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*> s;
        TreeNode *p=root;
        vector<int> result;
        while(p!==NULL||!s.empty()){
            while(p!=NULL){
                s.push(p);
                p=p->left;
            }
            if(!s.empty()){
                p=s.top();
                result.push_back(p->val);
                s.pop();
                p=p->right;
            }
        }
        return result;
    }

迭代方法