欢迎大家来到IT世界,在知识的湖畔探索吧!

• 前序遍历（中左右）：5 4 1 2 6 7 8
• 中序遍历（左中右）：1 4 2 5 7 6 8
• 后序遍历（左右中）：1 2 4 7 8 6 5
• 层序遍历：5 4 6 1 2 7 8

1、前序遍历

class Solution {public:    void dfs(TreeNode *cur, vector<int> &vec) {        if (cur == NULL) return;        vec.push_back(cur->val);  // 中        dfs(cur->left, vec);      // 左        dfs(cur->right, vec);     // 右    }    vector<int> preorderTraversal(TreeNode *root) {        vector<int> res;        dfs(root, res);        return res;    }};

欢迎大家来到IT世界,在知识的湖畔探索吧!

2、中序遍历

欢迎大家来到IT世界,在知识的湖畔探索吧!class Solution {
public:
    void dfs(TreeNode *cur, vector<int> &vec) {
        if (cur == NULL) return;
        dfs(cur->left, vec);
        vec.push_back(cur->val);
        dfs(cur->right, vec);
    }

    vector<int> inorderTraversal(TreeNode *root) {
        vector<int> res;
        dfs(root, res);
        return res;
    }
};

3、后序遍历

class Solution {
public:
    void dfs(TreeNode *cur, vector<int> &vec) {
        if (cur == NULL) return;
        dfs(cur->left, vec);
        dfs(cur->right, vec);
        vec.push_back(cur->val);
    }

    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> res;
        dfs(root, res);
        return res;
    }
};

4、层序遍历

每一层放入一个 vector 写法，如：[[3],[9,20],[15,7]]

欢迎大家来到IT世界,在知识的湖畔探索吧!class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode *root) {
        vector<vector<int>> res;
        if (!root) return res;

        queue<TreeNode *> q;
        q.push(root);
        while (!q.empty()) {
            int layerSz = q.size();
            vector<int> layer;
            for (int i = 0; i < layerSz; i++) {
                auto node = q.front();
                q.pop();
                layer.push_back(node->val);
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
            }
            res.push_back(layer);
        }
        return res;
    }
};

全部放一起，如：[3 9 20 15 7 ]

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode *root) {
        vector<int> res;
        queue<TreeNode *> q;
        q.push(root);
        while (!q.empty()) {
            auto node = q.front();
            q.pop();
            res.push_back(node->val);
            if (node->left) q.push(node->left);
            if (node->right) q.push(node->right);
        }
        for (const auto &c : res) cout << c << " ";
        return res;
    }
};