Given a string, return the characters that occurs only once.
hello
The character that occurs only once in the input string 'hello' is 'h', 'e' and 'o'. So the output will be:
`heo`
Complete the following task based on the instruction and optional input provided.### Instruction:
Given a string, return the characters that occurs only once.

### Input:
hello

### Output:
The character that occurs only once in the input string 'hello' is 'h', 'e' and 'o'. So the output will be:
`heo`