1. 先序遍历的非递归实现
1 vector<int> pre_order(TreeNode* root){
2 vector<int> res;
3 stack<TreeNode* > st;
4 if(root!=NULL) st.push(root);
5 while(!st.empty()){
6 TreeNode* node = st.top();
7 st.pop();
8 res.push_back(node->val);
9 if(node->right != NULL) st.push(node->right);
10 if(node->left != NULL) st.push(node->left);
11 }
12 return res;
13 }
2. 中序遍历的非递归实现
1 vector<int> in_order(TreeNode* root){
2 vector<int> res;
3 stack<TreeNode*> st;
4 if(root != NULL) st.push(root);
5 TreeNode* cur =root;
6 while(!st.empty()){
7 // 如果cur为空，弹出栈顶元素，并将cur设置为弹出节点的右孩子
8 if(cur == NULL){
9 res.push_back(st.top()->val);
10 cur = st.top()->right;
11 st.pop();
12 }else // 否则继续找左孩子
13 cur = cur->left;
14 if(cur != NULL)
15 st.push(cur);
16 }
17 return res;
18 }
3. 后序遍历的非递归实现
// 后序遍历的非递归实现(双栈实现)
vector<int> post_order(TreeNode* root){
vector<int> res;
stack<TreeNode*> s1;
stack<int> s2;
if(root!=NULL) s1.push(root);
while(!s1.empty()){
TreeNode* node = s1.top();
s1.pop();
s2.push(node->val);
if(node->left != NULL) s1.push(node->left);
if(node->right != NULL) s1.push(node->right);
}
while(!s2.empty()){
res.push_back(s2.top());
s2.pop();
}
return res;
}
1 // 后序遍历的非递归实现（单个栈实现）
2 vector<int> post_order_2(TreeNode* root){
3 vector<int> res;
4 stack<TreeNode*> st;
5 TreeNode* last = root, *cur = NULL;
6 if(root != NULL) st.push(root);
7 while(!st.empty()){
8 cur = st.top();
9 // 如果左子树没遍历结束
10 if(cur->left!=NULL && last!=cur->left && last!=cur->right)
11 st.push(cur->left);
12 else if(cur->right!=NULL && last!=cur->right)
13 st.push(cur->right);
14 else{
15 last = cur;
16 res.push_back(st.top()->val);
17 st.pop();
18 }
19 }
20 return res;
21 }
