java笔试常见的算法题

package com.wsl.test; import java.util.HashMap; import java.util.Map; import java.util.Stack; import java.util.StringTokenizer; public class StrOperation { public static void main(String[] args) { /* * String str = "abcdrt"; StrReverse1(str); * StrReverse2(str.toCharArray(), 0, str.length() - 1); */ // FirstNoRepeated(); // test(); charCounter(); } private static void StrReverse1(String str) { StringBuilder sb = new StringBuilder(); for (int i = str.length() - 1; i >= 0; i--) { sb.append(str.charAt(i)); } System.out.println("orignal:" + str + " reverse：" + sb.toString()); } private static void StrReverse2(char[] b, int start, int end) { char temp; while (start < end) { temp = b[end]; b[end] = b[start]; b[start] = temp; start++; end--; } System.out.print(new String(b)); } private static String StrReverse3(String s) { char[] str = s.toCharArray(); Stack<Character> stack = new Stack<Character>(); for (int i = 0; i < str.length; i++) { stack.push(str[i]); } StringBuilder reverse = new StringBuilder(); while (!stack.isEmpty()) { reverse.append(stack.pop()); } return reverse.toString(); } private static void StrReverse4(String s) { String reverse = new StringBuffer(s).reverse().toString(); } /** * 字符串替换函数 * * @param from * 要替换的字符 * @param to * 要替换成的目标字符 * @param source * 要替换的字符串 * @return 替换后的字符串 */ private static String str_replace1(String from, String to, String source) { StringBuffer sf = new StringBuffer(""); StringTokenizer st = new StringTokenizer(source, from, true); while (st.hasMoreTokens()) { String tmp = st.nextToken(); if (tmp.equals(from)) { sf.append(to); } else { sf.append(tmp); } } return sf.toString(); } private static String str_replace2(String source, String from, String to) { int j = 0; int i = 0; String result = ""; String temp = source; int fromlegth = from.length(); while (i < source.length()) { if (source.substring(i).startsWith(from)) { temp = source.substring(j, i) + to; result += temp; i += fromlegth; j = i; } else { i++; } } result += source.substring(j); return result; } private static String str_replace3(String source, String from, String to) { if (from == null || from.equals("")) { return source; } String result = ""; int formlength = from.length(); int intPos; while ((intPos = source.indexOf(from)) != -1) { result = result + source.substring(0, intPos); result = result + to; source = source.substring(intPos + formlength); } result = result + source; return result; } /** * 高效替换程序 没看懂 * * @return */ private static String str_replace4(String source, String from, String to) { if (source == null) { return null; } int i = 0; if ((i = source.indexOf(from, i)) >= 0) { char[] csrc = source.toCharArray(); char[] cto = to.toCharArray(); int len = from.length(); StringBuffer buf = new StringBuffer(source.length()); buf.append(csrc, 0, i).append(cto); i += len; int j = i; while ((i = source.indexOf(from, i)) > 0) { buf.append(csrc, j, i - j).append(cto); i += len; j = i; } buf.append(csrc, j, csrc.length - j); return buf.toString(); } return source; } // 最后一个字符特殊处理 private static Character FirstNoRepeated() { String str = "aabcb"; Character ch = null; for (int i = 0; i < str.length() - 1; i++) { if (str.split(String.valueOf(str.charAt(i))).length == 2 && str.charAt(i) != str.charAt(str.length() - 1)) { ch = new Character(str.charAt(i)); break; } } // 判断最后一个是否 if (str.charAt(str.length() - 1) != str.charAt(str.length() - 2) && str.split(String.valueOf(str.charAt(str.length() - 1))).length == 1) { ch = str.charAt(str.length() - 1); } if (ch != null) { System.out.println(ch); } return ch; } // 正规的办法 private static void FirstNoRepeated2() { String str = "wangshaolei"; Character ch; Map<Character, Integer> map = new HashMap<Character, Integer>(); for (int i = 0; i < str.length(); i++) { if (map.containsKey(str.charAt(i))) { map.put(str.charAt(i), 2); } else { map.put(str.charAt(i), 1); } } for (int i = 0; i < str.length(); i++) { if (map.get(str.charAt(i)) == 1) { ch = str.charAt(i); break; } } } /** * 统计每个字符出现的次数 */ private static void charCounter() { String str = "asdfasdfwerqwesdlfjgsldf啦啦啦啦大小写都行，中文也行，字母也行！"; Map<Character, Integer> counterMap = new HashMap<Character, Integer>(); for (char c : str.toCharArray()) { int count = 1; if (counterMap.containsKey(c)) { count += counterMap.get(c); } counterMap.put(c, count); } for (Map.Entry<Character, Integer> entry : counterMap.entrySet()) { System.out.println(entry.getKey() + ":" + entry.getValue()); } } }