DataStructure_Algorithm_HandBook_PreForLeetCode

617. Merge Two Binary Trees

You are given two binary trees root1 and root2.

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 the two trees 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 the new tree.

Return the merged tree.

Note: The merging process must start from the root nodes of both trees.

Example 1:

Input: root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
Output: [3,4,5,5,4,null,7]

Example 2:

Input: root1 = [1], root2 = [1,2]
Output: [2,2]

Constraints:

• The number of nodes in both trees is in the range [0, 2000].
• -104 <= Node.val <= 104

var mergeTrees = function(root1, root2) {
    if(root1 === null && root2 === null) return null; 
    //做一个处理，只要两个不是都为空，就可以累加起来 
    let num1 = root1 === null ? 0 : root1.val ;
    let num2 = root2 === null ? 0 : root2.val ;
    let node = new TreeNode(num1+num2);
    // 递归构建左右子树，注意这里要判断一下 null ，如果是空就传空就行了 
    node.left = mergeTrees(root1 === null ? null : root1.left, root2 === null ? null : root2.left);
    node.right = mergeTrees(root1 === null ? null : root1.right, root2 === null ? null : root2.right);
    return node;
};