计算连续子向量的最大和

package test;

import org.junit.Test;
public class FindGreatestSumOfSubArray {
    int maxNum = Integer.MIN_VALUE;

    /**
     * 基于递归写法,时间复杂度o(n)
     * 
     * @param i
     * @param a
     * @return
     */
    public int FindGreatestSumOfSubArray(int i, int[] a) {
        if (i == 0)
            return a[i];
        else {
            int tmp = FindGreatestSumOfSubArray(i - 1, a) + a[i];
            if (tmp <= 0) {
                return a[i];
            } 
            if (tmp <a[i]){
                tmp = a[i];
            }
            if (tmp > maxNum) {
                maxNum = tmp;
            }
            return tmp;
        }
    }

    /**
     * 递归方法的调用
     * 
     * @param i
     * @param a
     * @return
     */
    public int FindGreatestSumOfSubArrayRecru(int[] a) {
        int result = FindGreatestSumOfSubArray(a.length - 1, a);
        if (result < maxNum)
            return maxNum;
        else
            return result;
    }

    /**
     * 暴力搜索的方法，时间复杂度o(n^2)
     * @param a
     * @return
     */
    public int FindGreatestSumOfSubArrayViolent(int[] a) {
        // 暴力搜索
        int max = 0;
        for (int i = 0; i < a.length; i++) {
            int result = a[i];
            for (int j = i + 1; j < a.length; j++) {
                result += a[j];
                if (result > max) {
                    max = result;
                }
            }
        }
        return max;
    }
    
    /**
     * 动态规划写法,时间复杂度o(n)，时间复杂度O（1）
     * @param a
     * @return
     */
    public int FindGreatestSumOfSubArrayDP(int[] a) {
        //基于数组特性,也叫动态规划
             if (a == null || a.length == 0)return 0;
            int max = Integer.MIN_VALUE;
             int result = 0;
             for(int i=0;i<a.length;i++){
                /* if (result <= 0){
                     result = a[i];
                 }else{
                     result += a[i];
                 }
                 if (result > max){
                     max = result;
                 }*/
                 //if,else的另一种写法：
                 result = (result < 0)?a[i]:a[i]+result;
                 max = (result > max)?result:max;
             }
             return max;
    }
    
    @Test
    public void test() {
        int[] a = { 1, -2, 3, 10, -4, 7, 2, -5 };
        int result1 = FindGreatestSumOfSubArrayRecru(a);
        System.out.println(result1); //18
        int result2 = FindGreatestSumOfSubArrayDP(a);
        System.out.println(result2);//18
        int result3 = FindGreatestSumOfSubArrayViolent(a);
        System.out.println(result3);//18
    }
    
}