[LeetCode]101. Symmetric Tree

https://leetcode.com/problems/symmetric-tree/

判断二叉树是否轴对称

几乎全部树的问题一定有递归解法！！！

本题中需要递归比较的是：左子树的左子树和右子树的右子树 && 左子树的右子树和右子树的左子树

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        return root == null || partition(root.left, root.right);
    }
    private boolean partition(TreeNode left, TreeNode right) {
        if (left == null || right == null) {
            return left == right;
        }
        if (left.val != right.val) {
            return false;
        }
        return partition(left.left, right.right) && partition(left.right, right.left);
    }
}

非递归解法注意右子树插入的时候先插右子节点，再插左子节点，保证始终左子节点在前

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        List<TreeNode> left = new LinkedList();
        List<TreeNode> right = new LinkedList();
        left.add(root.left);
        right.add(root.right);
        while (left.size() != 0 && right.size() != 0) {
            int size = left.size();
            for (int i = 0; i < size; i++) {
                TreeNode nodeLeft = left.get(0);
                TreeNode nodeRight = right.get(right.size() - 1);
                if (nodeLeft == nodeRight || (nodeLeft != null && nodeRight != null && nodeLeft.val == nodeRight.val)) {
                    left.remove(0);
                    if (nodeLeft != null) {
                        left.add(nodeLeft.left);
                        left.add(nodeLeft.right);
                    }
                    right.remove(right.size() - 1);
                    if (nodeRight != null) {
                        right.add(0, nodeRight.right);
                        right.add(0, nodeRight.left);
                    }
                } else {
                    return false;
                }
            }
        }
        return left.size() == right.size();
    }
}