LeetCode--Symmetric Tree
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Problem:
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
Note: Bonus points if you could solve it both recursively and
iteratively. confused what “{1,#,2,3}” means? > read more on how
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Analysis:
方法1:再次创建一个新的函数,该函数有两个参数,一个是左节点,另一个是右节点,利用这两个记录的节点,分别判断左节点的左孩子?=右节点的右孩子;左节点的右孩子?=右节点的左孩子,将判断的结果返回;
这里其实的对两棵子树同时进行递归判断。
方法2:利用堆栈将节点按照特点的顺序(左节点压左孩子,右节点就得压右孩子;左节点压右孩子,右节点就得压左孩子)压入堆栈,把相应的节点作比较,不相同则返回false,相同的话就continue或者压入堆栈。
Anwser1:
递归:
public class Solution { public boolean isSymmetric(TreeNode root) { //recursively if(root==null) return true; else return isSymmetric(root.left,root.right); } public boolean isSymmetric(TreeNode leftNode,TreeNode rightNode){ if(leftNode==null && rightNode==null) return true; else if (leftNode==null || rightNode==null) return false; else { if (leftNode.val==rightNode.val) { boolean leftSym = isSymmetric(leftNode.left,rightNode.right); boolean rightSym = isSymmetric(leftNode.right,rightNode.left); return leftSym && rightSym; } else return false; } } }
Anwser2:
迭代
public class Solution { public boolean isSymmetric(TreeNode root) { if(root==null) return true; Stack<TreeNode> leftStack = new Stack<TreeNode>(); Stack<TreeNode> rightStack = new Stack<TreeNode>(); leftStack.push(root.left); rightStack.push(root.right); while(leftStack.size()>0 && rightStack.size()>0){ TreeNode leftNode = leftStack.pop(); TreeNode rightNode = rightStack.pop(); if(leftNode==null && rightNode==null) continue;//不能返回true,因为还有其他没有比较的节点。 else if(leftNode==null || rightNode==null) return false; else if(leftNode.val==rightNode.val){ //压入堆栈的顺序要注意 leftStack.push(leftNode.left); leftStack.push(leftNode.right); rightStack.push(rightNode.right); rightStack.push(rightNode.left); } else return false;//考虑的是leftNode.val!=rightNode.val的情况,不能丢 } return true;//考虑的是完全对称的树,最后应该返回true,不能丢 } }
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