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;
    }
};