BT_LevelOrder_It.cpp

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if(!root) return res;
        //We can use the length of the tree to indicate the elements in this level
        //A common trick, remember
        queue<TreeNode*> Q;
        Q.push(root);
        while(!Q.empty()){
            int len = Q.size();
            vector<int> tempStack;
            for(int i = 0; i < len; i++){
                TreeNode* cur = Q.front();
                Q.pop();
                tempStack.push_back(cur->val);
                if(cur->left) Q.push(cur->left);
                if(cur->right) Q.push(cur->right);
            }
            res.push_back(tempStack);
        }
        return res;
    }
};