【leetcode】Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

1
/ \
2   2
/ \ / \
3  4 4  3

But the following is not:

1
/ \
2   2
\   \
3    3

思路：判断树是否是对称的 递归判断即可

class Solution {
public:
bool isSymmetric(TreeNode* root) {
if(NULL == root)   return true;

TreeNode * L = root->left;
TreeNode * R = root->right;
return isSymmetricPart(L, R);
}

bool isSymmetricPart(TreeNode* L, TreeNode* R)
{
if(NULL == L && NULL == R) return true;
if(NULL == L && NULL != R || NULL == R && NULL != L) return false;
if(L->val == R->val)
return isSymmetricPart(L->left, R->right) && isSymmetricPart(L->right, R->left);
else
return false;

}
};

更简短的写法

class Solution {
public:
bool isSymmetric(TreeNode* root) {
return !root ? true : isSymmetricHelper(root->left, root->right);
}

bool isSymmetricHelper(TreeNode* left, TreeNode* right) {
if (!left && !right) { return true; }
return
(left && right) &&
(left->val == right->val) &&
isSymmetricHelper(left->left, right->right) &&
isSymmetricHelper(left->right, right->left);
}
};