二叉树专题

二叉树的前序遍历：
/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
非递归：
class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in vector which contains node values.
     */
    vector<int> preorderTraversal(TreeNode *root) {
        // write your code here
        auto R = root;
        //int top = -1;
        while((S.empty() == false) || R!= NULL){
            
            if(R != NULL){
                result.push_back(R->val);
                S.push(R);
                R = R->left;
            }else{
                R = S.top();
                S.pop();
                R=R->right;
            }
            
        }
        
        return result;
        
    }
    
    vector<int> result={};
    stack<TreeNode *> S;
};