Best Lemon Pie

Ingredients :

• 1 cup sugar
• 1/4 cup cornstarch
• 3 tablespoons all-purpose flour
• 1/4 teaspoon salt
• 2 cups water
• 3 large egg yolks, beaten
• 1 tablespoon butter
• 1/4 cup lemon juice
• 1 teaspoon grated lemon zest
• 1 pastry shell (9 inches), baked
• MERINGUE:
• 3 large egg whites
• 1/4 teaspoon salt
• 1/2 cup sugar