牛客高频题--树的层次遍历

NC15 求二叉树的层序遍历

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > levelOrder(TreeNode* root) {
        // write code here
        if(root==nullptr) return vector<vector<int>>{};
        // 层次遍历用队列而不用vector是由于queue比较少占用内存
        queue<TreeNode*> tq;tq.push(root);
        vector<int> lc={1};
        //vector<vector<TreeNode*>> tres={vector<TreeNode*>{root}};
        vector<vector<int>> res={vector<int>{root->val}};
        while(!tq.empty()){    // ...
            int prelev=lc.back();
            vector<int> tmp; int nextlev=0;
            for(int i=0;i<prelev;++i){
                // 父出队，子入队
                TreeNode* curN=tq.front();tq.pop();
                if(curN->left){
                    tq.push(curN->left);
                    nextlev++;
                    tmp.push_back(curN->left->val);
                } 
                if(curN->right){
                    tq.push(curN->right);
                    nextlev++;
                    tmp.push_back(curN->right->val);
                } 
            }
            if(!tmp.empty()){
                res.push_back(tmp);lc.push_back(nextlev);
            }
        }
        return res;
    }
};