栈和递归的关系 144:Binary Tree Preorder Traversal

//用栈来实现非递归解法
/**
 * Definition for a binary tree node.
 * 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) {
        vector<int> res;
        if(root == NULL)
            return res;
        stack<TreeNode*> stack;
        stack.push(root);
        
        while(!stack.empty()){
            TreeNode* c = stack.top();
            stack.pop();
            res.push_back(c->val);
            if(c->right)
                stack.push(c->right);
            if(c->left)
                stack.push(c->left);
        }
        return res;
    }
};

//递归解法，注意res是全局的，要放在遍历函数外面
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> res;
    vector<int> preorderTraversal(TreeNode* root) {
        
        if(root){
            res.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        return res;
    }
};

/**
 * Definition for a binary tree node.
 * 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;
        if(root == NULL) return res;  //若根节点为空则返回空
        TreeNode* p = root;
        while(p || !st.empty()){
            while(p){
                //先将根节点入栈，再将它所有的左节点入栈
                st.push(p);
                p = p->left;   
            }
            
            p = st.top();
            st.pop();
            res.push_back(p->val);
            p = p->right;
        }
        return res;
    }
};