[Leetcode]Binary Tree Iterative Traversal

Three problems of binary tree traversal of Leetcode:

Pre-order: http://oj.leetcode.com/problems/binary-tree-preorder-traversal/

In-order: http://oj.leetcode.com/problems/binary-tree-inorder-traversal/

Post-order: http://oj.leetcode.com/problems/binary-tree-postorder-traversal/

Pre-order Traversal

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<int> preorderTraversal(TreeNode *root) {        stack<TreeNode*> st;        vector<int> res;        TreeNode *now = root;                while (now || !st.empty()) {            if (!now) {                now = st.top();                st.pop();            }                        while (now) {                res.push_back(now -> val);                st.push(now -> right);                now = now -> left;            }        }        return res;    }};

In-order Traversal:

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<int> inorderTraversal(TreeNode *root) {         stack<TreeNode*> st;         vector<int> res;         TreeNode *p = root;                  while (p || !st.empty()) {             while (p) {                 st.push(p);                 p = p -> left;             }             if (!st.empty()) {                 TreeNode *now = st.top();                 st.pop();                 res.push_back(now -> val);                 p = now -> right;             }         }         return res;    }};

Post-order Traversal:

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<int> postorderTraversal(TreeNode *root) {        stack<TreeNode*> st;        vector<int> res;                if (!root) return res;                TreeNode *pre = NULL;        st.push(root);                while (!st.empty()) {            TreeNode *cur = st.top();            if ( (!cur -> left && !cur -> right) ||                    (pre && (pre == cur -> left || pre == cur -> right))) {                        res.push_back(cur -> val);                        st.pop();                        pre = cur;            } else {                if (cur -> right) st.push(cur -> right);                if (cur -> left)  st.push(cur -> left);            }        }        return res;    }};