Binary Tree Inorder Traversal: Given a binary tree, return the inorder traversal of its nodes’ values. Example: Input: [1,null,2,3] Output: [1,3,2]

Solution:

#include <iostream>
#include <stack>
#include <vector>
#include <string>

using namespace std;

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

struct Command{
    string s;
    TreeNode* node;
    Command(string s, TreeNode* node): s(s), node(node){}
};

// 中序遍歷二叉樹的非遞迴實現
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {

        vector<int> res;
        if ( root == NULL )
            return res;

        stack<Command> stack;
        stack.push( Command("go", root) );
        while ( !stack.empty() ){

            Command command = stack.top();
            stack.pop();

            if( command.s == "print" )
                res.push_back( command.node -> val );
            else{
                assert( command.s == "go" );
                if( command.node -> right )
                    stack.push( Command("go", command.node -> right) );
                stack.push( Command("print", command.node) );
                if( command.node -> left )
                    stack.push( Command("go", command.node -> left) );
            }
        }

        return res;

    }
};
 

總結： 上述解法是非遞迴的實現的中序遍歷，用了棧的思想，模擬系統棧的工作流程。遞迴方法簡單，就不實現了。