二叉树的遍历

二叉树的遍历通常有四种，前序遍历、中序遍历、后序遍历（分别以当前节点被操作的时机命名）和层序遍历

前序遍历

• 递归写法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        traversal(root, res);
        return res;
    }
    void traversal(TreeNode* root, vector<int>& vec) {
        if (!root)
            return;
        vec.push_back(root->val);
        traversal(root->left, vec);
        traversal(root->right, vec);
    }
};

• 迭代写法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode*> stk;
        vector<int> res;
        if (!root)
            return res;
        stk.push(root);
        while (!stk.empty()) {
            auto node = stk.top();
            res.push_back(node->val);
            stk.pop();
            if (node->right)
                stk.push(node->right);
            if (node->left)
                stk.push(node->left);
        }
        return res;
    }
};

中序遍历

• 递归写法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        traversal(root, res);
        return res;
    }
    void traversal(TreeNode* node, vector<int>& vec) {
        if (!node)
            return;
        traversal(node->left, vec);
        vec.push_back(node->val);
        traversal(node->right, vec);
    }
};

• 迭代写法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> stk;
        if (!root)
            return res;
        TreeNode* cur = root;
        while (cur || !stk.empty()) {
            if (cur) {
                stk.push(cur);
                cur = cur->left;
            } else {
                cur = stk.top();
                stk.pop();
                res.push_back(cur->val);
                cur = cur->right;
            }
        }
        return res;
    }
};

后序遍历

• 递归写法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> res;
        travsersal(root, res);
        return res;
    }
    void travsersal(TreeNode* root, vector<int>& vec) {
        if (!root)
            return;
        travsersal(root->left, vec);
        travsersal(root->right, vec);
        vec.push_back(root->val);
    }
};

• 迭代写法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        stack<TreeNode*> stk;
        vector<int> res;
        if (!root)
            return res;
        stk.push(root);
        while (!stk.empty()) {
            auto node = stk.top();
            stk.pop();
            res.push_back(node->val);
            if (node->left)
                stk.push(node->left);
            if (node->right)
                stk.push(node->right);
        }
        return vector<int>(res.rbegin(), res.rend());
    }
};

层序遍历

• 递归写法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        order(root, res, 0);
        return res;
    }
    void order(TreeNode* node, vector<vector<int>>& res, int depth) {
        if (!node)
            return;
        if (depth == res.size())
            res.push_back(vector<int>());
        res[depth].push_back(node->val);
        order(node->left, res, depth + 1);
        order(node->right, res, depth + 1);
    }
};

• 迭代写法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        queue<TreeNode*> que;
        vector<vector<int>> res;
        if (!root)
            return res;
        que.push(root);
        while (!que.empty()) {
            int sz = que.size();
            vector<int> vec;
            for (int i = 0; i < sz; ++i) {
                TreeNode* node = que.front();
                que.pop();
                vec.push_back(node->val);
                if (node->left)
                    que.push(node->left);
                if (node->right)
                    que.push(node->right);
            }
            res.push_back(vec);
        }
        return res;
    }
};