[leetcode] Symmetric Tree

通过preorder产生一个list, 含有String = val + "," + level
然后判断是否对称就好了

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public class Solution { ArrayList<String> list = new ArrayList<String>(); public boolean isSymmetric(TreeNode root) { if (root == null) return true; inOrder(root, 0); if (list.size() % 2 == 0) return false; int i = 0; int j = list.size()-1; while (i != j){ if (!list.get(i).equals(list.get(j))){ return false; } i++; j--; } return true; } private void inOrder(TreeNode root, int level){ if (root == null) return; inOrder(root.left, level+1); String temp = root.val+","+level; list.add(temp); inOrder(root.right, level+1); } }

另一种方法... 每次都没有反应过来。。。其实细心点就会发现要将left.left 和right.right 对比还有left.right和right.left对比

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/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public boolean isSymmetric(TreeNode root) { if (root == null) return true; return check(root.left, root.right); } boolean check(TreeNode left, TreeNode right){ if (left == null && right == null) return true; if ((left != null && right == null)|| (left == null && right != null)|| (left.val != right.val)) return false; return check(left.left, right.right)&&check(left.right, right.left); } }