问为什么决策树边界是方形的，而支持向量机是圆形/椭圆形的？

EN

from sklearn.linear_model import LogisticRegression
from sklearn.ensemble import GradientBoostingClassifier
from sklearn.svm import SVC

import pandas
import numpy

# Read the data
data = pandas.read_csv('data.csv')

# Split the data into X and y
X = numpy.array(data[['x1', 'x2']])
y = numpy.array(data['y'])

# import statements for the classification algorithms
from sklearn.linear_model import LogisticRegression
from sklearn.ensemble import GradientBoostingClassifier
from sklearn.svm import SVC

    # Logistic Regression Classifier
    classifier = LogisticRegression()
    classifier.fit(X,y)

    # Decision Tree Classifier
    classifier = GradientBoostingClassifier()
    classifier.fit(X,y)

    # Support Vector Machine Classifier
    classifier = SVC()
    classifier.fit(X,y)