[LeetCode] 100. Same Tree Java

题目：

Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value.、

题意及分析：给出两棵树，判断两棵树是否完全相同，结构和值都相同。有两种方法，一种是遍历树，对每次的节点做判断，比较复杂；另一种是使用递归，每次对左右子树进行判断。

第一种方法：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {
        if((p==null&&q!=null)||(p!=null&&q==null)) return false;
        if(p==null&&q==null) return true;
        Stack<TreeNode> pStack = new Stack<>();
        Stack<TreeNode> qStack = new Stack<>();
        TreeNode pNode = p;
        TreeNode qNode = q;
        pStack.add(pNode);
        qStack.add(qNode);
        while(pNode.left!=null||!pStack.empty()){       //对q做和p一样的操作，如果得到的值不一样那么就返回false
            if(pNode.left!=null){
                if(qNode.left!=null){
                    qNode = qNode.left;
                    qStack.add(qNode);
                }else{
                    return false;   //结构不相等返回false
                }
                pNode = pNode.left;
                pStack.add(pNode);
            }else{
                if(qNode.left!=null) return  false;     //如果p屋左子节点而q还有 那么返回false
                if(qStack.isEmpty()) return false;     //结构不相等返回false
                TreeNode pNow = pStack.pop();
                TreeNode qNow = qStack.pop();
                if(pNow.val!=qNow.val) return false;        //不相等 直接返回false
                if(pNow.right!=null){
                    if(qNow.right==null) return false;
                    pNode=pNow.right;
                    qNode=qNow.right;
                    pStack.add(pNode);
                    qStack.add(qNode);
                }else{      //pNode没有右节点，但是q有,fanhui false
                    if(qNow.right!=null) return false;
                }

            }
        }
        return true;
    }
}

第二种方法：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {
        if((p==null&&q!=null)||(p!=null&&q==null)) return false;
        if(p==null&&q==null) return true;
        if(p.val == q.val)
            return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
        return false;
    }
}