树结构习题练习

1，判断二叉树是否对称

递归：

基本想法就是在函数中传入一边的左，和一边的右……

isSymmetric(leftNode.left, rightNode.right)

isSymmetric(leftNode.right, rightNode.left)

以这样的递归来不断比较。

public class SymmetricTree {
    static class TreeNode {
           int val;
            TreeNode left;
            TreeNode right;
            TreeNode(int x) { val = x; }
        }
    public boolean isSymmetric(TreeNode root) {
        if(root == null) return true;
        return isSymmetric(root.left, root.right);
    }
    public boolean isSymmetric(TreeNode leftNode, TreeNode rightNode){
        if(leftNode == null && rightNode == null ) return true;
        if(leftNode == null || rightNode == null) return false;
        if(leftNode.val != rightNode.val) return  false;
        boolean isLeft = isSymmetric(leftNode.left, rightNode.right);
        boolean isRight = isSymmetric(leftNode.right, rightNode.left);
        return isLeft && isRight;
    }

    public static void main(String args[]){
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(2);
        root.left.left = new TreeNode(3);
        root.left.right = new TreeNode(4);
        root.right.left = new TreeNode(4);
        root.right.right = new TreeNode(3);
        System.out.println(new SymmetricTree().isSymmetric(root));
        System.out.println(new SymmetricTree().isSymmetricIter(root));
    }
}

　　非递归的思路，就是分别遍历左右两边。只不过是反着次序遍历。

每次都各自出栈后往各自栈中放左右节点。一边是先放左后放右，一边是先放右后放左。

public boolean isSymmetricIter(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 left = leftStack.pop();
            TreeNode right = rightStack.pop();
            if(left == null && right == null) continue;
            if(left == null || right == null) return false;
            if(left.val == right.val) {
                leftStack.push(left.right);
                leftStack.push(left.left);
                rightStack.push(right.left);
                rightStack.push(right.right);
            }else{
                return false;
            }
        }
        return true;
    }