Tree[0]

Tree Traversal – preorder

recursive

class Solution {public:    vector<int> preorderTraversal(TreeNode *root) {        vector<int> rs;        visit(rs, root);        return rs;    }    void visit(vector<int> &rs, TreeNode *root)    {        if (root == NULL) return;        rs.push_back(root->val);        visit(rs, root->left);        visit(rs, root->right);    }};

iterative

class Solution {public:    vector<int> preorderTraversal(TreeNode *root) {        vector<int> rs;        if (!root) return rs;        stack<TreeNode*> st;        st.push(root);        while(!st.empty()) {            TreeNode *node = st.top();            st.pop();            rs.push_back(node->val);            if (node->right) st.push(node->right);            if (node->left) st.push(node->left);        }        return rs;    }};

Tree Traversal – inorder

iterative
thoughts:

1. Create an empty stack S.
2. Initialize current node as root
3. Push the current node to S and set current = current->left until current is NULL

4. If current is NULL and stack is not empty then
   a) Pop the top item from stack.
   b) Print the popped item, set current = popped_item->right
   c) Go to step 3.

5. If current is NULL and stack is empty then we are done.

class Solution {public:    vector<int> inorderTraversal(TreeNode *root) {        /*         * 处理完左子树，处理根节点，然后处理右子树         */        vector<int> rs;        if (!root) return rs;        stack<TreeNode *> st;        st.push(root);        TreeNode *p = root->left;  //当前要处理的节点        while (!st.empty() || p != NULL) {            if (p != NULL) {                  st.push(p);                p = p->left;            } else {                TreeNode *top = st.top();                st.pop();                rs.push_back(top->val);                p = top->right;            }        }        return rs;    }};

Tree Traversal – postorder

iterative

class Solution {public:    vector<int> postorderTraversal(TreeNode *root) {        vector<int> rs;        if (!root) return rs;        stack<TreeNode *> s1;        stack<TreeNode *> s2;        s1.push(root);        while (!s1.empty()) {            TreeNode *node = s1.top();            s1.pop();            s2.push(node);            if (node->left) s1.push(node->left);            if (node->right) s1.push(node->right);        }        while(!s2.empty()) {            rs.push_back(s2.top()->val);            s2.pop();        }        return rs;    }};