leetcode 第144题，第94题，第145题，第102题

前序遍历
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> ans;
        if(root==NULL) return ans;
        //栈的初始化
        TreeNode *stack[10000];
        int top=-1;
        TreeNode *p=root;
        //p==NULL同时栈中没有元素时跳出循环
        while(top!=-1||p){
            if(p){
                ans.push_back(p->val);  //这句话只能加在p=p->left或p=p->right的上面一句
                stack[++top] = p;
                p=p->left;
            }
            else{
                p = stack[top];
                top--;
                p=p->right;
            }
        }
        return ans;
    }
};

中序遍历
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> ans;
        TreeNode *stack[100001];
        int top=-1;
        TreeNode *p = root;
        while(top!=-1||p){
            if(p){
                stack[++top] = p;
                p=p->left;
            }
            else{
                p = stack[top];
                top--;
                ans.push_back(p->val);
                p=p->right;
            }
        }
        return ans;
    }
};

后序遍历
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ans;
        TreeNode *stack[100001];
        int top=-1;
        TreeNode *p = root;
        while(top!=-1||p){
            if(p){
                ans.push_back(p->val);
                stack[++top] = p;
                p=p->right;
            }
            else{
                p = stack[top];
                top--;
                p=p->left;
            }
        }
        reverse(ans.begin(), ans.end());
        return ans;

    }
};

层序遍历
/**
 * 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:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        //队列的初始化
        TreeNode *q[10000];
        int front=0,rear=0;
        if(root==NULL) return ans;
        q[rear++] = root;
        while(front!=rear){
            int len = rear-front;
            vector<int> level;
            for(int i=0;i<len;i++){
                TreeNode *temp = q[front++];
                if(temp->left!=NULL) q[rear++] = temp->left;
                if(temp->right!=NULL) q[rear++] = temp->right;
                level.push_back(temp->val);
            }
            ans.push_back(level);
        }
        return ans;
    }
};