Revise Tree With One Zero

题目：
一个完全树，node的值为0或者1，每个parent的val是两个child ‘and’的结果，现在把一个leaf flip一下(0->1 or 1->0)，把tree修正一遍。

解题思路：
这道题主要用bottom-up的方法来解决。首先找到那个node，然后往上一步一步修正。

代码：

public class ReviseTreeWithOneZero {

    static class TreeNode{
        int val;
        TreeNode left;
        TreeNode right;
        boolean flipped = false;
        public TreeNode(int val){
            this.val = val;
        }
    }

    public static int revise(TreeNode root){
        if(root.left == null && root.right == null){
            if(root.flipped){
                return root.val ^ 1;
            }
            else{
                return root.val;
            }
        }
        int left = revise(root.left);
        int right = revise(root.right);
        return left & right;
    }

    public static void print(TreeNode root){
        if(root == null){
            return;
        }
        print(root.left);
        System.out.println(root.val);
        print(root.right);
    }

    public static void main(String[] args){
        TreeNode node4 = new TreeNode(0);
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(1);
        node2.flipped = true;
        TreeNode node3 = new TreeNode(1);
        TreeNode node5 = new TreeNode(0);
        TreeNode node6 = new TreeNode(0);
        TreeNode node7 = new TreeNode(1);
        node3.right = node2;
        node3.left = node1;
        node6.left = node5;
        node6.right = node7;
        node4.left = node3;
        node4.right = node6;

        System.out.println("before: ");
        print(node4);
        revise(node4);
        System.out.println("after: ");
        print(node4);
    }

}