LeetCode:Symmetric Tree(循环版)

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
bool isSymmetric(TreeNode* root) {
stack<TreeNode*> sTl,sTr;
TreeNode* l = root,*r = root;
while(l || r || !sTl.empty() || !sTr.empty()){
if(l || r){
if( l == nullptr || r == nullptr || l->val != r->val ) return false;
sTl.push(l);
sTr.push(r);
l = l->left;
r = r->right;
}
else {
TreeNode* tL = sTl.top();
TreeNode* tR = sTr.top();
if(tL->val != tR->val )  return false;
sTl.pop();
sTr.pop();
l = tL->right;
r = tR->left;
}
}
return true;
}
};