617. merge binary tree

subject ：

Answer key ：

Ideas ：（1） recursive （2） The tree is traversed in sequence , Set up null The node of is 0, Add two numbers

Code ：

// recursive 
var mergeTrees = function (t1, t2) {
    if (t1 === null) return t2;
    if (t2 === null) return t1;
    t1.val += t2.val;
    t1.left = mergeTrees(t1.left, t2.left);
    t1.right = mergeTrees(t1.right, t2.right);
    return t1;
};

（ps： Fifth day ）