Solution

• 层序遍历

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/

class Solution {
public:
    vector<vector<int> > Print(TreeNode* pRoot) {
        vector<vector<int>> res;
        if (pRoot == nullptr) return res;
        TreeNode* cur = pRoot;
        queue<TreeNode *> que;
        que.push(cur);
        bool flag = true; // 是否正序
        while (!que.empty()) {
            int n = que.size();
            vector<int> level;
            for (int i = 0; i < n; ++i) {
                cur = que.front();
                que.pop();
                level.push_back(cur->val);
                if (cur->left) que.push(cur->left);
                if (cur->right) que.push(cur->right);
            }
            if (!flag)
                reverse(level.begin(), level.end());
            res.push_back(level);
            flag = !flag;
        }
        return res;
    }
};