最大子序列（java版)

package com.algorithm.test;
/**
 * 最大子序列
 * @author LiFen
 *
 */
public class LargestSubsequence {
    public static void main(String[] args) {
        int[] arr = {4,-3,5,-2,-1,2,6,-2};
        System.out.println(maxSubSum(arr));
        
        System.out.println(maxSumRec(arr, 0, arr.length - 1));
        
        System.out.println(maxSubSum2(arr));
    }
    
    /*
     * 时间复杂度O(N*N)
     */
    public static int maxSubSum(int [] a) {
        int maxSum = 0;
        
        for(int i = 0; i < a.length; i++) {
            int thisSum = 0;
            for(int j = 0; j < a.length; j++) {
                
                thisSum += a[j];
                
                if(thisSum > maxSum)
                    maxSum = thisSum;
            }
        }
        return maxSum;
    }
    /*
     * 递归
     * 时间复杂度O(N logN)
     */
    public static int maxSumRec(int [] a, int left, int right) {
        if(left == right) {
            if(a[left] > 0)
                return a[left];
            else return 0;
        }
        
        int center = (left + right) / 2;
        int maxLeftSum = maxSumRec(a, left, center);
        int maxRightSum = maxSumRec(a, center + 1, right);
        
        int maxLeftBorderSum = 0,leftBorderSum = 0;
        for(int i = center; i >= left; i--) {
            leftBorderSum += a[i];
            if(leftBorderSum > maxLeftBorderSum)
                maxLeftBorderSum = leftBorderSum;
        }
        
        int maxRightBorderSum = 0, rightBoderSum = 0;
        for(int i = center + 1; i <= right; i++) {
            rightBoderSum += a[i];
            if(rightBoderSum > maxRightBorderSum)
                maxRightBorderSum = rightBoderSum;
        }
        return Math.max(Math.max(maxLeftSum, maxRightSum),maxLeftBorderSum + maxRightBorderSum);
    }
    /*
     * 联机算法
     * 快
     */
    public static int maxSubSum2(int [] a) {
        int maxSum = 0, thisSum = 0;
        
        for(int i = 0; i < a.length; i++) {
            thisSum += a[i];
            
            if(thisSum > maxSum) {
                maxSum = thisSum;
            }else if(thisSum < 0){
                thisSum = 0;
            }
        }
        return maxSum;
    }
}