二叉树的非递归遍历（先序， 中序， 后序）

先序遍历：

/**
 * 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> result;
        if(root != NULL)
            st.push(root);
        else
            return result;
        while(!st.empty()){
            TreeNode *node = st.top();
            st.pop();
            result.push_back(node->val);
            if(node->right != NULL)
                st.push(node->right);
            if(node->left != NULL)
                st.push(node->left);
        }
        return result;
    }
};

中序遍历：

/**
 * 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) {
        vector<int> result;
        stack<TreeNode *> st;
        if(root == NULL)
            return result;
        if(root->right != NULL)
            st.push(root->right);
        st.push(root);
        TreeNode* node = root->left;
        while(node != NULL){
            if(node->right != NULL)
                st.push(node->right);
            st.push(node);
            node = node->left;
        }
        while(!st.empty()){
            node = st.top();
            st.pop();
            result.push_back(node->val);
            if(!st.empty() && st.top() == node->right){
                node = st.top();
                st.pop();
                while(node != NULL){
                    if(node->right != NULL)
                        st.push(node->right);
                    st.push(node);
                    node = node->left;
                }
            }
        }
        return result;
    }
};

后序遍历：

/**
 * 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) {
        vector<int> result;
        stack<TreeNode *> st;
        TreeNode *node = root;
        if(node == NULL)
            return result;
        st.push(node);
        if(node->right != NULL)
            st.push(node->right);
        if(node->left != NULL)
            st.push(node->left);
        TreeNode *lastNode = NULL;
        while(!st.empty()){
            node = st.top();
            st.pop();
            if(node->left == NULL && node->right == NULL){
                result.push_back(node->val);
                lastNode = node;
            }
            else if(lastNode != NULL && (lastNode == node->left || lastNode == node->right)){
                result.push_back(node->val);
                lastNode = node;
            }
            else {
                st.push(node);
                if(node->right != NULL)
                st.push(node->right);
                if(node->left != NULL)
                    st.push(node->left);
            }
        }
        return result;
    }
};