二叉树四种遍历方式

二叉树前序遍历

递归深度搜索方法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> ret;
    vector<int> preorderTraversal(TreeNode* root) {
        if(root){
            ret.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        return ret;
    }
};

迭代深度搜索方法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> ret;
        stack<TreeNode*> st;
        TreeNode*cur=root;
        while(!st.empty()||cur)
        {
            while(cur)
            {
                ret.push_back(cur->val);
                st.push(cur);
                cur=cur->left;
            }
            cur=st.top();
            st.pop();
            cur=cur->right;
        }
        return ret;
    }
};

LeetCode

二叉树中序遍历

递归深度搜索方法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> ret;
    vector<int> inorderTraversal(TreeNode* root) {
        if(root){
            inorderTraversal(root->left);
            ret.push_back(root->val);
            inorderTraversal(root->right);
        }
        return ret;
    }
};

迭代深度搜索方法

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

LeetCode

二叉树后续遍历

递归深度搜索方法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> ret;
    vector<int> postorderTraversal(TreeNode* root) {
        if(root)
        {
            postorderTraversal(root->left);
            postorderTraversal(root->right);
            ret.push_back(root->val);
        }
        return ret;
    }
};

后续遍历LeetCode标记为困难，主要是这里需要标记节点的右节点是否被访问过。如果当前节点的右节点被访问过或者没有右节点，那就可以访问当前节点了。

迭代深度搜索方法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ret;
        stack<TreeNode*> st;
        TreeNode* cur=root;
        TreeNode* pre=nullptr;
        while(!st.empty()||cur)
        {
            while(cur)
            {
                st.push(cur);
                cur=cur->left;
            }
            cur=st.top();
            if(cur->right==nullptr||pre==cur->right)//右节点已经遍历
            {
                st.pop();
                ret.push_back(cur->val);
                pre=cur;
                cur=nullptr;
            }
            else
            {
                cur=cur->right;
                pre=nullptr;
            }
        }
        return ret;
    }
};

LeetCode

二叉树层续遍历

递归深度搜索方法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        pre(root,0,res);
        return res;
    }
    void pre(TreeNode *root,int depth,vector<vector<int>> &res)
    {
        if(!root) return;
        if(depth==res.size())   //说明需要增加一层
            res.push_back(vector<int>{});
        res[depth].push_back(root->val);
        pre(root->left,depth+1,res);
        pre(root->right,depth+1,res);
    }
};

迭代广度搜索方法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        queue<TreeNode*> qu;
        vector<vector<int>> ret;
        if(!root)    return ret;
        qu.push(root);
        while(!qu.empty())
        {
            int len=qu.size();//这一层的元素个数
            vector<int> level;
            for(int i=0;i<len;i++)
            {
                TreeNode* node=qu.front();
                level.push_back(node->val);
                qu.pop();
                if(node->left)  qu.push(node->left);
                if(node->right) qu.push(node->right);
            }
            ret.push_back(level);
        }
        return ret;
    }
};

LeetCode