lintcode，等价二叉树

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检查两棵二叉树是否等价。等价的意思是说，首先两棵二叉树必须拥有相同的结构，并且每个对应位置上的节点上的数都相等。

一刷ac
解题思路：用递归和非递归分别实现。都是细节问题，递归判断两个节点是否全不为空，是否值相等，一集左右子树是否等价。非递归用两个栈存两个数的节点，每次存放一个节点，并判断另一个树能不能做相同的存放。
递归实现

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @param a, b, the root of binary trees.     * @return true if they are identical, or false.     */    public boolean isIdentical(TreeNode a, TreeNode b) {        if(a == null && b == null) return true;        if(a == null || b == null) return false;        if(a.val != b.val) return false;        return isIdentical(a.left, b.left) && isIdentical(a.right, b.right);    }}

非递归实现。

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @param a, b, the root of binary trees.     * @return true if they are identical, or false.     */    public boolean isIdentical(TreeNode a, TreeNode b) {        if(a == null && b == null) return true;        if(a == null || b == null) return false;         Stack<TreeNode> stacka = new Stack<TreeNode>();        Stack<TreeNode> stackb = new Stack<TreeNode>();        stacka.push(a);        stackb.push(b);        while(!stacka.empty() && !stackb.empty()){            TreeNode nodea = stacka.pop();            TreeNode nodeb = stackb.pop();            if(nodea.val != nodeb.val) return false;            if(nodea.left != null){                if(nodeb.left == null) return false;                if(nodea.left.val == nodeb.left.val){                    stacka.push(nodea.left);                    stackb.push(nodeb.left);                }else return false;            }            if(nodea.right != null){                if(nodeb.right == null) return false;                if(nodea.right.val == nodeb.right.val){                    stacka.push(nodea.right);                    stackb.push(nodeb.right);                }else return false;            }        }        return stacka.empty() && stackb.empty();    }}

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