代码随想录算法训练营第十四天 | 二叉树● 理论基础● 递归遍历 ● 迭代遍历● 统一迭代

● 递归遍历

前序遍历

/**
 * 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* cur,vector<int>& vec){
        if(cur==nullptr){
            return ;
        }
        // vec.push_back((*cur).val);
        vec.push_back(cur->val);
        traversal(cur->left,vec);
        traversal(cur->right,vec);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> result;
        traversal(root,result);
        return result;
    }
};

后序遍历

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

中序遍历

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

● 迭代遍历

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> result;
        stack<TreeNode*> st;
        if(root==nullptr) return result;
        st.push(root);
        while(!st.empty()){
            TreeNode* node = st.top();
            result.push_back(node->val);
            st.pop();
            if(node->right) st.push(node->right);
            if(node->left) st.push(node->left);
        }
        return result;
    }
};