62 lines
2.0 KiB
Java
62 lines
2.0 KiB
Java
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/*
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Output the first character which occurred once in the string.
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The input string can consist of any characters, like %,? or ü.
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There are no constraints. For examples, see the main function.
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Time Complexity: O(n)
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Explanation of the algorithm:
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Iterating through the string twice:
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First time, the program counts occurrences into a HashMap.
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Second time, the program looks for the first character
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in the string with the HashMap value of 1. */
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import java.util.*;
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public class FirstNonRepeatingChar {
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// Contains examples of usage of the algorithm
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public static void main(String[] args) {
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String example1 = "abcbadabfaa334";
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System.out.println(firstNonRepeatingChar(example1));
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// Output: c
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String example2 = "aab!b?cc!dü";
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System.out.println(firstNonRepeatingChar(example2));
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// Output: ?
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String example3 = "aa!b?cc!ddb?";
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System.out.println(firstNonRepeatingChar(example3));
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// Output: null
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String example4 = "üaa55vv22??1.";
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System.out.println(firstNonRepeatingChar(example4));
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// Output: ü
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}
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/*
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Return the first character in string a that only occurs in string a once.
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If every character in the string occurs more than once, return null.
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*/
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public static Character firstNonRepeatingChar(String a) {
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// map characters to the number of occurrences in the string
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HashMap<Character, Integer> count = new HashMap<>();
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// To iterate through the string, turn it to an array of characters
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char[] charArray = a.toCharArray();
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// Count occurrences of each character
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for (Character i : charArray) {
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Integer value = count.putIfAbsent(i, 0);
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if (value == null) count.put(i, 1);
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else count.put(i, value + 1);
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}
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// Search for the first matching character
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for (Character k : charArray) {
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if (count.get(k) == 1) {
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return k;
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}
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}
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return null;
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}
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}
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