LeetCode 144，94，145：二叉树三种深度优先遍历

递归：

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        if(!root) return res;

        preorder(root, res);
        return res;
    }

    void preorder(TreeNode* root, vector<int> &res) {
        if(!root) return;
        res.push_back(root->val);
        if(root->left) preorder(root->left, res);
        if(root->right) preorder(root->right, res);
    }
};

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        if(!root) return res;

        inorder(root, res);
        return res;
    }

    void inorder(TreeNode* root, vector<int>& res) {
        if(!root) return;
        if(root->left) inorder(root->left, res);
        res.push_back(root->val);
        if(root->right) inorder(root->right, res);
    }
};

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> res;
        if(!root) return res;

        postorder(root, res);
        return res;
    }

    void postorder(TreeNode* root, vector<int> &res) {
        if(!root) return;
        if(root->left) postorder(root->left, res);
        if(root->right) postorder(root->right, res);
        res.push_back(root->val);
    }
};

迭代：

先序

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        if(!root) return res;

        stack<TreeNode*> s;
        s.push(root);
        TreeNode* tmp;
        while(!s.empty()) {
            tmp = s.top();
            s.pop();
            res.push_back(tmp->val);

            if(tmp->right) s.push(tmp->right);
            if(tmp->left) s.push(tmp->left);
        }
        return res; 
    }
};

中序：

自己写的，但是内存超限，不清楚哪里有问题 （可能是重复访问，压栈过多）

    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        if(!root) return res;

        stack<TreeNode*> s;
        s.push(root);
        TreeNode* tmp;
        while(!s.empty()) {
            tmp = s.top();

            while(tmp->left) {
                s.push(tmp);
                tmp = tmp->left;
            }
            res.push_back(tmp->val);
            s.pop();

            if(tmp->right) s.push(tmp->right);
        }

        return res;
    }

参考别人的模板写的： 常用算法及参考实现

    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        if(!root) return res;

        stack<TreeNode*> s;
        TreeNode* tmp = root;
        while(!s.empty() || tmp) {
            while(tmp) {
                s.push(tmp);
                tmp = tmp->left;
            }
            tmp = s.top();
            s.pop();
            res.push_back(tmp->val);

            //if(tmp->right) s.push(tmp->right);
            tmp = tmp->right;
        }
        return res;
    }

后序：（比较复杂，有几个点需要注意的） 

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> path;
        unordered_set<TreeNode*> _set;
        TreeNode* node;
        
        if(root) path.push(root);

        while(!path.empty()) {
            node = path.top(); //只取出元素，不弹栈:只有访问过的节点才被弹出

            bool leftVisited = true, rightVisited = true;
            if(node->right && _set.find(node->right) == _set.end()) {
                rightVisited = false;
                path.push(node->right);
            }
            if(node->left && _set.find(node->left) == _set.end()) {
                leftVisited = false;
                path.push(node->left);
            }
            if(leftVisited && rightVisited) {
                res.push_back(node->val);
                path.pop();
                _set.insert(node);
            }
        }
        return res;
    }
};