递归及非递归二叉树遍历方法

三种递归遍历

//前序遍历
void preorder(TreeNode *root, vector<int> &path)
{
if(root != NULL)
{
path.push_back(root->val);
preorder(root->left, path);
preorder(root->right, path);
}
}

//中序遍历
void inorder(TreeNode *root, vector<int> &path)
{
if(root != NULL)
{
inorder(root->left, path);
path.push_back(root->val);
inorder(root->right, path);
}
}

//后续遍历
void postorder(TreeNode *root, vector<int> &path)
{
if(root != NULL)
{
postorder(root->left, path);
postorder(root->right, path);
path.push_back(root->val);
}
}

教科书上的非递归方法

//非递归前序遍历
void preorderTraversal(TreeNode *root, vector<int> &path)
{
stack<TreeNode *> s;
TreeNode *p = root;
while(p != NULL || !s.empty())
{
while(p != NULL)
{
path.push_back(p->val);
s.push(p);
p = p->left;
}
if(!s.empty())
{
p = s.top();
s.pop();
p = p->right;
}
}
}

//非递归中序遍历
void inorderTraversal(TreeNode *root, vector<int> &path)
{
stack<TreeNode *> s;
TreeNode *p = root;
while(p != NULL || !s.empty())
{
while(p != NULL)
{
s.push(p);
p = p->left;
}
if(!s.empty())
{
p = s.top();
path.push_back(p->val);
s.pop();
p = p->right;
}
}
}

//非递归后序遍历-迭代
void postorderTraversal(TreeNode *root, vector<int> &path)
{
stack<TempNode *> s;
TreeNode *p = root;
TempNode *temp;
while(p != NULL || !s.empty())
{
while(p != NULL) //沿左子树一直往下搜索，直至出现没有左子树的结点
{
TreeNode *tempNode = new TreeNode;
tempNode->btnode = p;
tempNode->isFirst = true;
s.push(tempNode);
p = p->left;
}
if(!s.empty())
{
temp = s.top();
s.pop();
if(temp->isFirst == true)   //表示是第一次出现在栈顶
{
temp->isFirst = false;
s.push(temp);
p = temp->btnode->right;
}
else  //第二次出现在栈顶
{
path.push_back(temp->btnode->val);
p = NULL;
}
}
}
}

简化统一格式的非递归算法

//更简单的非递归前序遍历
void preorderTraversalNew(TreeNode *root, vector<int> &path)
{
stack< pair<TreeNode *, bool> > s;
s.push(make_pair(root, false));
bool visited;
while(!s.empty())
{
root = s.top().first;
visited = s.top().second;
s.pop();
if(root == NULL)
continue;
if(visited)
{
path.push_back(root->val);
}
else
{
s.push(make_pair(root->right, false));
s.push(make_pair(root->left, false));
s.push(make_pair(root, true));
}
}
}

//更简单的非递归中序遍历
void inorderTraversalNew(TreeNode *root, vector<int> &path)
{
stack< pair<TreeNode *, bool> > s;
s.push(make_pair(root, false));
bool visited;
while(!s.empty())
{
root = s.top().first;
visited = s.top().second;
s.pop();
if(root == NULL)
continue;
if(visited)
{
path.push_back(root->val);
}
else
{
s.push(make_pair(root->right, false));
s.push(make_pair(root, true));
s.push(make_pair(root->left, false));
}
}
}

//更简单的非递归后序遍历
void postorderTraversalNew(TreeNode *root, vector<int> &path)
{
stack< pair<TreeNode *, bool> > s;
s.push(make_pair(root, false));
bool visited;
while(!s.empty())
{
root = s.top().first;
visited = s.top().second;
s.pop();
if(root == NULL)
continue;
if(visited)
{
path.push_back(root->val);
}
else
{
s.push(make_pair(root, true));
s.push(make_pair(root->right, false));
s.push(make_pair(root->left, false));
}
}
}