动态规划

1.求一个整数序列的最长递增子序列。
2.编辑距离算法。即从一个字符串转换成另一个字符串的最少操作次数。允许添加，删除或是替换字母。
3.两个整数数组，从每个数组中有序取m个数，两两相乘后的和最大。求最大和。
4.求两个字符串的最长公共子串。

java 代码

public int getDistance(String s1, String s2) {
       int[][] array = new int[s1.length() + 1][s2.length() + 1];
       for (int i = 0; i <= s1.length(); i++) {
           array[i][0] = i;
       }
       for (int j = 0; j <= s2.length(); j++) {
           array[0][j] = j;
       }
       for (int i = 1; i <= s1.length(); i++) {
           for (int j = 1; j <= s2.length(); j++) {
               array[i][j] = Integer.MAX_VALUE;
               array[i][j] = Math.min(array[i][j], array[i - 1][j] + 1);
               array[i][j] = Math.min(array[i][j], array[i][j - 1] + 1);
               int c = s1.charAt(i - 1) == s2.charAt(j - 1) ? 0 : 1;
               array[i][j] = Math.min(array[i][j], array[i - 1][j - 1] + c);
           }
       }
       return array[s1.length()][s2.length()];
   }

java 代码

public int getMaxMulti(int[] a, int[] b, int N) {
    int[] a1 = new int[a.length + 1];
    int[] a2 = new int[b.length + 1];
    for (int i = 0; i < a.length; i++) {
        a1[i + 1] = a[i];
    }
    for (int i = 0; i < b.length; i++) {
        a2[i + 1] = b[i];
    }
    int[][][] m = new int[N + 1][a1.length][a2.length];
    for (int k = 1; k <= N; k++) {
        for (int i = k; i < a1.length; i++) {
            for (int j = k; j < a2.length; j++) {
                int max = Integer.MIN_VALUE;
                if (i - 1 >= k) {
                    max = Math.max(max, m[k][i - 1][j]);
                }
                if (j - 1 >= k) {
                    max = Math.max(max, m[k][i][j - 1]);
                }
                max = Math.max(max, m[k - 1][i - 1][j - 1] + a1[i] * a2[j]);
                m[k][i][j] = max;
            }
        }
    }

    return m[N][a.length][b.length];
}  

java 代码

//最长公共字串
public class LongestCommonSequence {

    /**
     * @param args
     */
    public static void main(String[] args) {
        System.out
            .println(new LongestCommonSequence().getLCS("pailbyujcdfefdghijk", "abcaduefsgdhdijdfk"));

    }

    public String getLCS(String s1, String s2) {
        int[][] lens = new int[s1.length() + 1][s2.length() + 1];
        // 0 for [i][j], 1 for [i][j-1], -1 for [i-1][j]
        int[][] directions = new int[s1.length() + 1][s2.length() + 1];
        for (int i = 1; i <= s1.length(); i++) {
            for (int j = 1; j <= s2.length(); j++) {
                if (s1.charAt(i - 1) == s2.charAt(j - 1)) {
                    lens[i][j] = lens[i - 1][j - 1] + 1;
                    directions[i][j] = 0;
                } else if (lens[i][j - 1] >= lens[i - 1][j]) {
                    lens[i][j] = lens[i][j - 1];
                    directions[i][j] = 1;
                } else {
                    lens[i][j] = lens[i - 1][j];
                    directions[i][j] = -1;
                }
            }
        }

        return getCommon(s1, directions, s1.length(), s2.length());
    }

    private String getCommon(String s1, int[][] directions, int len1, int len2) {
        if (len1 == 0 || len2 == 0) {
            return "";
        }
        if (directions[len1][len2] == 0) {
            return getCommon(s1, directions, len1 - 1, len2 - 1) + s1.charAt(len1 - 1);
        } else if (directions[len1][len2] == 1) {
            return getCommon(s1, directions, len1, len2 - 1);
        } else {
            return getCommon(s1, directions, len1 - 1, len2);
        }
    }
}