立志重刷代码随想录60天冲冲冲！！——第十三天

144. 二叉树的前序遍历

前序：中左右

递归

/**
 * 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:
    // 确定传入参数、返回值
    void traversal (TreeNode* curr, vector<int>& vec) {
        // 确定终止条件
        if (curr == NULL) return;
        // 确定递归逻辑
        vec.push_back(curr->val);
        traversal(curr->left, vec);
        traversal(curr->right, vec);
    }

    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        traversal(root, res);
        return res;
    }
};

（迭代）

/**
 * 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) {
        // 栈存放结点
        stack<TreeNode*> st;
        vector<int> res;

        if (root == nullptr) return res;
        st.push(root);
        while (!st.empty()) {
            TreeNode* node = st.top();
            st.pop();
            res.push_back(node->val);

            // 前序：中左右。但是因为是栈，所以先放右边，后放左边
            if (node->right) st.push(node->right);
            if (node->left) st.push(node->left);
        }
        return res;
    }
};

145. 二叉树的后序遍历

前序：左右中

递归

/**
 * 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:
    void Traversal (TreeNode* curr, vector<int>& vec) {
        if (curr == nullptr) return;
        Traversal(curr->left, vec);
        Traversal(curr->right, vec);
        vec.push_back(curr->val);
    }

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

迭代

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        stack<TreeNode*> st;
        vector<int> res;

        if (root == nullptr) return res;
        st.push(root);
        while (!st.empty()) {
            TreeNode* node = st.top();
            st.pop();
            res.push_back(node->val);

            // 后序：左右中。先左后右，
            if (node->left) st.push(node->left);
            if (node->right) st.push(node->right);
        }
        // 再翻转！
        reverse(res.begin(), res.end());
        return res;
    }
};

94. 二叉树的中序遍历

递归

/**
 * 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:
    void Traversal (TreeNode* curr, vector<int>& vec) {
        if (curr == nullptr) return;
        Traversal(curr->left, vec);
        vec.push_back(curr->val);
        Traversal(curr->right, vec);
    }

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

102. 二叉树的层序遍历

重点记！！！！！！

/**
 * 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>> results;

        if (root != nullptr) que.push(root);
        while (!que.empty()) {
            int size = que.size();
            vector<int> vec;
            while (size--) {
                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);
            }
            results.push_back(vec);
        }
        return results;
    }
};

107. 二叉树的层序遍历 II

在最后反转数组就行

/**
 * 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>> levelOrderBottom(TreeNode* root) {
        queue<TreeNode*> que;
        vector<vector<int>> results;

        if (root != nullptr) que.push(root);
        while (!que.empty()) {
            int size = que.size();
            vector<int> vec;
            while (size--) {
                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);
            }
            results.push_back(vec);
        }
        reverse(results.begin(), results.end());
        return results;
    }
};