百度面试总结

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项目介绍
介绍一下linux命令
线程进程的区别
线程和进程共享什么资源
线程共享的环境包括：进程代码段、进程的公有数据(利用这些共享的数据，线程很容易的实现相互之间的通讯)、进程打开的文件描述符、信号的处理器、进程的当前目录和进程用户ID与进程组ID。
设计模式，单例双重检查其实并发有问题
大数加法（+-号）

    public class Main {    /**     * 两个大数相加     *     * @param a     * @param b     * @return     * @throws Exception     */    private static String Add(String a, String b) throws Exception {        // 处理空        if (a == null || b == null || "".equals(a) || "".equals(b)) {            throw new Exception("number is null.");        }        // 处理负号        boolean aIsNegative = false, bIsNegative = false;        if (a.startsWith("-")) {            aIsNegative = true;            a = a.substring(1);        }        if (b.startsWith("-")) {            bIsNegative = true;            b = b.substring(1);        }        // 处理正号        if (a.startsWith("+")) a = a.substring(1);        if (b.startsWith("+")) b = b.substring(1);        if (!check(a) || !check(b)) {            throw new Exception("number is error.");        }        if (aIsNegative == bIsNegative) {            String ans = AddCore(a, b);            if (aIsNegative) {                return "0".equals(ans) ? "0" : "-" + ans;            } else {                return ans;            }        } else {            if (!(a.length() > b.length() || a.length() == b.length() && a.compareTo(b) >= 0)) {                // swap                String stmp = a;                a = b;                b = stmp;                boolean btmp = aIsNegative;                aIsNegative = bIsNegative;                bIsNegative = btmp;            }            String ans = SubtractCore(a, b);            if (aIsNegative && !bIsNegative) {                // -2+1                return "0".equals(ans) ? "0" : "-" + ans;            } else {                // 2+-1                return ans;            }        }    }    /**     * 正数加法     *     * @param a     * @param b     * @return     */    private static String AddCore(String a, String b) throws Exception {        if (a == null || b == null || "".equals(a) || "".equals(b)) {            throw new Exception("number is null.");        }        int maxx = Math.max(a.length(), b.length());        StringBuilder asb = new StringBuilder(a);        for (int i = a.length(); i < maxx; i++) {            asb.insert(0, '0');        }        a = asb.toString();        StringBuilder bsb = new StringBuilder(b);        for (int i = b.length(); i < maxx; i++) {            bsb.insert(0, '0');        }        b = bsb.toString();        StringBuilder ans = new StringBuilder();        int e = 0;        for (int i = maxx - 1; i >= 0; i--) {            e = e + (a.charAt(i) - '0') + (b.charAt(i) - '0');            ans.insert(0, e % 10);            e /= 10;        }        while (e != 0) {            ans.insert(0, e % 10);            e /= 10;        }        String c = handlePrefixZero(ans.toString());        return "".equals(c) || c.length() == 0 ? "0" : c;    }    /**     * 参数为a>b     * 正数减法，返回结果为大的数减小的数     *     * @param a     * @param b     * @return     */    private static String SubtractCore(String a, String b) throws Exception {        if (a == null || b == null || "".equals(a) || "".equals(b)) {            throw new Exception("number is null.");        }        int maxx = Math.max(a.length(), b.length());        StringBuilder asb = new StringBuilder(a);        for (int i = a.length(); i < maxx; i++) {            asb.insert(0, '0');        }        a = asb.toString();        StringBuilder bsb = new StringBuilder(b);        for (int i = b.length(); i < maxx; i++) {            bsb.insert(0, '0');        }        b = bsb.toString();        StringBuilder ans = new StringBuilder();        int e = 0;        for (int i = maxx - 1; i >= 0; i--) {            e = (e + (a.charAt(i) - '0')) - (b.charAt(i) - '0');            if (e < 0) {                ans.insert(0, e + 10);                e = -1;            } else {                ans.insert(0, e);                e = 0;            }        }        String c = handlePrefixZero(ans.toString());        return c;    }    /**     * 正数乘法     *     * @param a     * @param b     * @return     */    private static String MultCore(String a, int b) throws Exception {        if (a == null || "".equals(a)) {            throw new Exception("number is null.");        }        if (b == 0) return "0";        StringBuilder ans = new StringBuilder();        int e = 0;        for (int i = a.length() - 1; i >= 0; i--) {            e = e + (a.charAt(i) - '0') * b;            ans.insert(0, e % 10);            e /= 10;        }        while (e != 0) {            ans.insert(0, e % 10);            e /= 10;        }        String c = handlePrefixZero(ans.toString());        return "".equals(c) || c.length() == 0 ? "0" : c;    }    /**     * 处理前缀0     *     * @param c     * @return     */    private static String handlePrefixZero(String c) {        if (c.startsWith("0")) {            int pos = 0;            while (pos < c.length() && c.charAt(pos) == '0') {                pos++;            }            c = c.substring(Math.min(pos, c.length() - 1));        }        return c;    }    /**     * 判断是否为数字     *     * @param s     * @return     */    private static boolean check(String s) {        if (s == null || "".equals(s)) return false;        for (int i = 0, len = s.length(); i < len; i++) {            char ch = s.charAt(i);            if (ch >= '0' && ch <= '9') continue;            else return false;        }        return true;    }    /**     * 大整数乘法     *     * @param a     * @param b     * @return     */    private static String Mult(String a, String b) throws Exception {        // 处理空        if (a == null || b == null || "".equals(a) || "".equals(b)) {            throw new Exception("number is null.");        }        // 处理负号        boolean aIsNegative = false, bIsNegative = false;        if (a.startsWith("-")) {            aIsNegative = true;            a = a.substring(1);        }        if (b.startsWith("-")) {            bIsNegative = true;            b = b.substring(1);        }        // 处理正号        if (a.startsWith("+")) a = a.substring(1);        if (b.startsWith("+")) b = b.substring(1);        if (!check(a) || !check(b)) {            throw new Exception("number is error.");        }        if (!(a.length() > b.length() || a.length() == b.length() && a.compareTo(b) >= 0)) {            // swap            String stmp = a;            a = b;            b = stmp;            boolean btmp = aIsNegative;            aIsNegative = bIsNegative;            bIsNegative = btmp;        }        String ans = "0";        for (int i = b.length() - 1, j = 0; i >= 0; i--, j++) {            StringBuilder tmp = new StringBuilder(MultCore(a, b.charAt(i) - '0'));            for (int k = 0; k < j; k++) {                tmp.append("0");            }            ans = Add(ans, tmp.toString());        }        if (aIsNegative == bIsNegative) return ans;        else return "-" + ans;    }    /**     * 大整数减法     *     * @param a     * @param b     * @return     */    private static String Subtract(String a, String b) throws Exception {        // 处理空        if (a == null || b == null || "".equals(a) || "".equals(b)) {            throw new Exception("number is null.");        }        // 处理负号        boolean aIsNegative = false, bIsNegative = false;        if (a.startsWith("-")) {            aIsNegative = true;            a = a.substring(1);        }        if (b.startsWith("-")) {            bIsNegative = true;            b = b.substring(1);        }        // 处理正号        if (a.startsWith("+")) a = a.substring(1);        if (b.startsWith("+")) b = b.substring(1);        if (!check(a) || !check(b)) {            throw new Exception("number is error.");        }        if (!(a.length() > b.length() || a.length() == b.length() && a.compareTo(b) >= 0)) {            // swap            String stmp = a;            a = b;            b = stmp;            boolean btmp = aIsNegative;            aIsNegative = bIsNegative;            bIsNegative = btmp;        }        if (aIsNegative && bIsNegative) {            // 2-1            return SubtractCore(a, b);        } else if (aIsNegative && !bIsNegative) {            // 2-(-1)=2+1            return AddCore(a, b);        } else if (!aIsNegative && bIsNegative) {            // -2-1 = -(2+1)            String ans = AddCore(a, b);            return "0".equals(ans) ? "0" : "-" + ans;        } else {            //-2-(-1)=-2+1=-(2-1)            String ans = SubtractCore(a, b);            return "0".equals(ans) ? "0" : "-" + ans;        }    }    public static void main(String[] args) throws Exception {        System.out.println(111*(-12));        System.out.println(Mult("111", "-12"));    }}

怎么才能最小次数使一个字符串变成回文的字符串

import java.util.Scanner;public class Main {    private static int dp[][] = null;    private static int dp(char chs[], int start, int end) {        if (chs == null || chs.length == 0                || start >= chs.length || end >= chs.length                || start >= end) {            return 0;        }        if (chs[start] == chs[end]) {            return dp[start][end] = dp(chs, start + 1, end - 1);        } else {            int leftPop = -1, rightPop = -1;            if (dp[start + 1][end] == -1) dp[start + 1][end] = dp(chs, start + 1, end);            if (dp[start][end - 1] == -1) dp[start][end - 1] = dp(chs, start, end - 1);            leftPop = dp[start + 1][end];            rightPop = dp[start][end - 1];            return dp[start][end] = Math.min(leftPop, rightPop) + 1;        }    }    private static void init(int dp[][]) {        for (int i = 0, len = dp.length; i < len; i++) {            for (int j = i; j < dp[0].length; j++) {                dp[i][j] = dp[j][i] = -1;            }        }    }    public static void main(String[] args) {        Scanner cin = new Scanner(System.in);        while (cin.hasNext()) {            int n = cin.nextInt();            for (int i = 0; i < n; i++) {                String s = cin.next();                dp = new int[s.length()][s.length()];                init(dp);                System.out.println(dp(s.toCharArray(), 0, s.length() - 1));            }        }    }}

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