【1错-1】Construct String from Bina

https://leetcode.com/problems/construct-string-from-binary-tree/

日期	是否一次通过	comment
2019-01-20 02:38	否	对何时加()理解不够清楚

image.png image.png

（来源：https://leetcode.com/problems/construct-string-from-binary-tree/）

1. 非递归：TODO；
2. 递归：preOrder，处理好何时加上()

1. 非递归

2.递归

class Solution {
    public String tree2str(TreeNode t) {
        if(t == null) {
            return "";
        }
        
        StringBuilder sb = new StringBuilder();
        helper(t, sb);
        return sb.toString();
    }
    
    private void helper(TreeNode t, StringBuilder sb) {
        if(t == null) {
            return;
        }
        sb.append(t.val);
        if(t.left != null || t.right != null) {
            sb.append("(");
            helper(t.left, sb);
            sb.append(")");
            
            if(t.right != null) {
                sb.append("(");
                helper(t.right, sb);
                sb.append(")");
            }
        }
        
    }
}