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
Java Solution – Recursion
This problem can be solve by using a simple recursion. The key is finding the conditions that return false, such as value is not equal, only one node(left or right) has value.
public boolean isSymmetric(TreeNode root) { if (root == null) return true; return isSymmetric(root.left, root.right); } public boolean isSymmetric(TreeNode l, TreeNode r) { if (l == null && r == null) { return true; } else if (r == null || l == null) { return false; } if (l.val != r.val) return false; if (!isSymmetric(l.left, r.right)) return false; if (!isSymmetric(l.right, r.left)) return false; return true; } |
private static boolean isSymmetric(Node a,Node b){
if(a==null && b==null) return true;
if(a==null || b==null) return false;
return a.val==b.val && isSymmetric(a.left,b.right) && isSymmetric(a.right,b.left);
}
public class leet_func {
String tree1 = “”;
String tree2 = “”;
public void PostOrder(TreeNode root){
if(root == null){
tree1 += null;
return;
}
PostOrder(root.left);
PostOrder(root.right);
//System.out.println(root.val);
tree1 += root.val;
}
public void reversePostOrder(TreeNode root){ //Reverse of post order to compare
if(root == null){ //Comparison should give the same result.
tree2+=null;
return;
}
reversePostOrder(root.right);
reversePostOrder(root.left);
//System.out.println(root.val);
tree2 += root.val;
}
public boolean isSymmetric(TreeNode root){
PostOrder(root);
reversePostOrder(root);
System.out.println(tree1);
System.out.println(tree2);
//Comparison gives the same result. Draw the tree and check
//PostOrder -> left -> right -> root
//ReversePostOrder -> right -> left -> root
if(tree1.equals(tree2)){
return true;
}else{
return false;
}
}
}
Iterative solution. We will need two queue two store the left and right.
if(root == null) return true;
Queue left = new LinkedList();
Queue right = new LinkedList();
left.add(root.left);
right.add(root.right);
while(!left.isEmpty() && !right.isEmpty()){
TreeNode l = left.remove();
TreeNode r = right.remove();
if(l == null && r == null) continue;
else if(l == null || r == null) return false;
if(l.val != r.val){
return false;
}
else{
left.add(l.left);
left.add(l.right);
right.add(r.right);
right.add(r.left);
}
}
return true;