Smallest String Starting From Leaf

题目连结

1.先移路往左走(直到左边是null)，删掉当下的点(往回走)，再往右下去
2.如果右边也走到null那就回到在上面的点

code

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode() {} *     TreeNode(int val) { this.val = val; } *     TreeNode(int val, TreeNode left, TreeNode right) { *         this.val = val; *         this.left = left; *         this.right = right; *     } * } */class Solution {    String ans="";     private void bfs(TreeNode root ,StringBuffer s){        if(root == null)return;        s.append((char)('a'+root.val));        if(root.left == null && root.right == null){               String tem = s.reverse().toString();            s.reverse();            if(ans=="" || ans.compareTo(tem)>0){                ans = tem;            }        }        bfs(root.right,s);        bfs(root.left,s);        s.deleteCharAt(s.length()-1);    }    public String smallestFromLeaf(TreeNode root) {        bfs(root,new StringBuffer());        return ans;    }}