fd

Author: dingjikerbo

leetcode/solution/src/SymmetricTree.java

@@ -1,3 +1,7 @@
+import java.util.LinkedList;
+import java.util.Queue;
+import java.util.Stack;
+
 public class SymmetricTree {
 
     public boolean isSymmetric(TreeNode root) {
@@ -16,4 +20,72 @@ private boolean isSymmetric(TreeNode left, TreeNode right) {
         }
         return left.val == right.val && isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);
     }
+
+    public boolean isSymmetric2(TreeNode root) {
+        if (root == null) {
+            return true;
+        }
+        Queue<TreeNode> queue1 = new LinkedList<>();
+        Queue<TreeNode> queue2 = new LinkedList<>();
+        queue1.offer(root.left);
+        queue2.offer(root.right);
+        while (!queue1.isEmpty() && !queue2.isEmpty()) {
+            TreeNode left = queue1.poll();
+            TreeNode right = queue2.poll();
+            if (left == null && right == null) {
+                continue;
+            }
+            if (left == null || right == null) {
+                return false;
+            }
+            if (left.val != right.val) {
+                return false;
+            }
+            queue1.offer(left.left);
+            queue1.offer(left.right);
+            queue2.offer(right.right);
+            queue2.offer(right.left);
+        }
+        return queue1.isEmpty() && queue2.isEmpty();
+    }
+
+    /**
+     * 非递归写法
+     */
+    public boolean isSymmetric2(TreeNode left, TreeNode right) {
+        if (left == null && right == null) {
+            return true;
+        }
+        if (left == null || right == null) {
+            return false;
+        }
+
+        Stack<TreeNode> stack1 = new Stack<TreeNode>();
+        Stack<TreeNode> stack2 = new Stack<TreeNode>();
+
+        while (!stack1.isEmpty() || left != null) {
+            if (left != null) {
+                if (right == null) {
+                    return false;
+                }
+                if (left.val != right.val) {
+                    return false;
+                }
+                stack1.push(left);
+                stack2.push(right);
+                left = left.left;
+                right = right.right;
+            } else {
+                if (right != null) {
+                    return false;
+                }
+                if (stack2.isEmpty()) {
+                    return false;
+                }
+                left = stack1.pop().right;
+                right = stack2.pop().left;
+            }
+        }
+        return stack2.isEmpty() && right == null;
+    }
 }

leetcode/src/Main.java

@@ -4,24 +4,48 @@
 
 public class Main {
 
-    public TreeNode trimBST(TreeNode root, int L, int R) {
-        if (root == null) {
-            return root;
+    public boolean isSymmetric(TreeNode left, TreeNode right) {
+        if (left == null && right == null) {
+            return true;
         }
-
-        if (root.val > R) {
-            root.right = null;
-            return trimBST(root.left, L, R);
+        if (left == null || right == null) {
+            return false;
         }
 
-        if (root.val < L) {
-            root.left = null;
-            return trimBST(root.right, L, R);
+        Stack<TreeNode> stack1 = new Stack<TreeNode>();
+        Stack<TreeNode> stack2 = new Stack<TreeNode>();
+
+        while (!stack1.isEmpty() || left != null) {
+            if (left != null) {
+                if (right == null) {
+                    return false;
+                }
+                if (left.val != right.val) {
+                    return false;
+                }
+                stack1.push(left);
+                stack2.push(right);
+                left = left.left;
+                right = right.right;
+            } else {
+                if (right != null) {
+                    return false;
+                }
+                if (stack2.isEmpty()) {
+                    return false;
+                }
+                left = stack1.pop().right;
+                right = stack2.pop().left;
+            }
         }
+        return stack2.isEmpty() && right == null;
+    }
 
-        root.left = trimBST(root.left, L, R);
-        root.right = trimBST(root.right, L, R);
-        return root;
+    public boolean isSymmetric(TreeNode root) {
+        if (root == null) {
+            return true;
+        }
+        return isSymmetric(root.left, root.right);
     }
 
     public static void main(String[] args) {