二叉树的遍历

#include <iostream>
#include <vector>
#include <stack>
using std::stack;
using std::vector;

struct TreeNode{
int val;
TreeNode* leftNode;
TreeNode* rightNode;
TreeNode(int x) : val(x),leftNode(nullptr),rightNode(nullptr) {}
};

class Solution {
public:
vector<int> preOrderTraversal(TreeNode* root) {
vector<int> res = {};
stack<TreeNode*> s;
if(root)  s.push(root);
TreeNode* p;
while(!s.empty())
{
p = s.top();
res.push_back(p->val);
s.pop();
if(p->rightNode)  s.push(p->rightNode);
if(p->leftNode) s.push(p->leftNode);
}
return res;
}

vector<int> inOrderTraversal(TreeNode* root)
{
vector<int> res = {};
stack<TreeNode*> s;
TreeNode* p = root;
while(!s.empty() || p)
{
if(p)
{
s.push(p);
p= p->leftNode;
}
else
{
p = s.top();
res.push_back(p->val);
s.pop();
p = p->rightNode;
}
}
return res;
}

vector<int> postOrderTraversal(TreeNode* root)
{
vector<int> res = {};
stack<TreeNode*> s;
TreeNode* p = root; //being access
TreeNode* q = nullptr; //previous access

do{
while(p)
{
s.push(p);
p = p->leftNode;
}
q = nullptr;
while(!s.empty())
{
p = s.top();
s.pop();
if(p->rightNode == q)
{
res.push_back(p->val);
q = p;
}
else
{
s.push(p);
p = p->rightNode;
break;
}
}
}while(!s.empty());
return res;
}

vector<vector<int>> levelOrderTraversal(TreeNode* root)
{
vector<vector<int>> res;
if(!root) return res;
levelOrderTraversalCore(root,1,res);
return res;
}
private:
void levelOrderTraversalCore(TreeNode* root,int level,vector<vector<int>>& res)
{
if(!root) return;
if(level > res.size())
res.push_back(vector<int>());
res[level-1].push_back(root->val);
levelOrderTraversalCore(root->leftNode,level+1,res);
levelOrderTraversalCore(root->rightNode,level+1,res);
}
};