• 递归是面试『树』时候永远滴神 
/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param t1 TreeNode类 
     * @param t2 TreeNode类 
     * @return TreeNode类
     */
    TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
        // write code here
        if( nullptr==t1 ) return t2;
        if( nullptr==t2 ) return t1;

        TreeNode * ret=new TreeNode( t1->val + t2->val );
        ret->left=mergeTrees(t1->left, t2->left);
        ret->right=mergeTrees( t1->right , t2->right );
        return ret;
    }
};