Merge Two Binary Trees–LeetCode#617

617.Merge Two Binary Trees

Given two binary trees and imagine that when you put one of them to cover the other, some nodes of the two trees are overlapped while the others are not.

You need to merge them into a new binary tree. The merge rule is that if two nodes overlap, then sum node values up as the new value of the merged node. Otherwise, the NOT null node will be used as the node of new tree.

(两个二叉树合并，节点相加)

Example:

Input:
	Tree 1                     Tree 2
          1                         2
         / \                       / \
        3   2                     1   3
       /                           \   \
      5                             4   7
Output:
Merged tree:
	     3
	    / \
	   4   5
	  / \   \
	 5   4   7

Note: The merging process must start from the root nodes of both trees.(必须从根结点开始合并)
思路：二叉树相加，可以用递归算法

public class MergeTwoBinaryTrees {
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1 == null) return t2;
        if (t2 == null) return t1;
        TreeNode result = new TreeNode(t1.val + t2.val);
        result.left = mergeTrees(t1.left, t2.left);
        result.right = mergeTrees(t1.right, t2.right);
        return result;
    }
}