814. Binary Tree Pruning(leetcode) (tree traverse)

https://leetcode.com/contest/weekly-contest-79/problems/binary-tree-pruning/ -- 814 from leetcode

tree traverse (post traverse) left right and root

the model

void traverse(node){
  if(node.left!=null) traverse(node.left);
  if(node.right!=null) traverse(node.right);
  //do something, current node is the deep leaf(left or right)
}

another model

int traverse(node){
  if(node.left!=null) l = traverse(node.left); // value is its left child
  if(node.right!=null) r = traverse(node.right); // value of its right child
  //do something, current node is the deep leaf(left or right)
  return node.val;
}

This problem we use second model and check if the children are null(tag ==2 : two children are nulls)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode pruneTree(TreeNode root) {
        int val = traverse(root);
        if(val == 0 && root.left==null && root.right==null) root=null;
        return root;
    }
    int traverse(TreeNode node){//when for
        //lright root
        int tag = 0;
        if(node.left!=null){
            int l = traverse(node.left);//retur the vale of next layer(left child)
            if(l==0) {
                node.left = null;
                tag ++;
            } 

        }else tag++;
        if(node.right!= null){
            int r = traverse(node.right);
            if(r== 0) {
                node.right = null;
                tag ++;
            }
        }else tag ++;
        //root
        if(node.val==0 && tag==2)//remove this node
            return 0;
        else return 1;
    }
}

ziji youdian cai(newbie) I only figured out one problem from the contest.