can’t read INPUT with PCF8575 when some IOs use for OUTPUT at the same time

can’t read INPUT with PCF8575 when some IOs use for OUTPUT at the same time

Hello, i have use the PCF8575, some GPIOs work for INPUT, some work for OUTPUT.
But now, i can’t read INPUT state. when the code Serial.println(p15Digital); always =0
how to solve it, thanks!

here is my code:

PCF8575 PCF8575_IN4(&I2Cone,0x26,4,16);

PCF8575_IN4.pinMode(0,INPUT);
PCF8575_IN4.pinMode(1,INPUT);
PCF8575_IN4.pinMode(2,INPUT);
PCF8575_IN4.pinMode(3,INPUT);
PCF8575_IN4.pinMode(4,INPUT);
PCF8575_IN4.pinMode(5,INPUT);
PCF8575_IN4.pinMode(6,INPUT);
PCF8575_IN4.pinMode(7,INPUT);

PCF8575_IN4.pinMode(8,OUTPUT); // for 74HC4067-A
PCF8575_IN4.pinMode(9,OUTPUT); // for 74HC4067-B
PCF8575_IN4.pinMode(10,OUTPUT); // for 74HC4067-C
PCF8575_IN4.pinMode(11,OUTPUT); // for 74HC4067-D

PCF8575_IN4.pinMode(12,OUTPUT); //LED-D7
PCF8575_IN4.pinMode(13,OUTPUT); //LED-D8

PCF8575_IN4.pinMode(14,INPUT); //Button-S3
PCF8575_IN4.pinMode(15,INPUT); //Button-S4

PCF8575_IN4.begin();

int p15Digital = PCF8575_IN4.digitalRead(15);
Serial.println(p15Digital);