872. Leaf-Similar Trees

Consider all the leaves of a binary tree.  From left to right order, the values of those leaves form a leaf value sequence.

For example, in the given tree above, the leaf value sequence is (6, 7, 4, 9, 8).

Two binary trees are considered leaf-similar if their leaf value sequence is the same.

Return true if and only if the two given trees with head nodes root1 and root2 are leaf-similar.

Note:

• Both of the given trees will have between 1 and 100 nodes

//Time: O(n), Space: O(logn) 其实是recusive stack的空间  
  public boolean leafSimilar(TreeNode root1, TreeNode root2) {
        if (root1 == null && root2 == null) {
            return true;
        }
        
        if (root1 == null || root2 == null) {
            return false;
        }
        
        StringBuilder s1 = new StringBuilder();//不可以用string因为recusive每次return又变成原来的string，所以string永远是初始值
        StringBuilder s2 = new StringBuilder();
        leaf(root1, s1);
        leaf(root2, s2);
        return s1.toString().equals(s2.toString());//note:stringbuilder用equal比较是错的，因为stringbuilder没有重写equal这个方法，所以equal本质仍然是==，比较的是地址
    }
    
    private void leaf(TreeNode root, StringBuilder sb) {
        if (root == null) {
            return;
        }
        
        if (root.left == null && root.right == null) {
            sb.append(root.val);
        }
        
        leaf(root.left, sb);
        leaf(root.right, sb);
    }