本次笔记内容:
1.3.1 应用实例_算法1&2
1.3.2 应用实例_算法3
1.3.3 应用实例_算法4
给定N个整数的序列 { A 1 , A 2 , . . . , A N } \{A_1,A_2,...,A_N\} {A1,A2,...,AN},求函数 f ( i , j ) = m a x 0 , ∑ k = 1 j A k f(i,j)=max{0,\sum^j_{k=1}A_k} f(i,j)=max0,∑k=1jAk的最大值。
int MaxSubseqSum1(int A[], int N)
{
int ThisSum, MaxSum = 0;
int i, j, k;
for (i = 0; i < N; i++)
{
for (j = 1; j < N; j++)
{
ThisSum = 0;
for (k = i; k <= j; k++)
ThisSum += A[k];
if (ThisSum > MaxSum)
MaxSum = ThisSum;
}
}
return MaxSum;
}
复杂度 T ( N ) = O ( N 3 ) T(N)=O(N^3) T(N)=O(N3)
int MaxSubseqSum2(int A[], int N)
{
int ThisSum, MaxSum = 0;
int i, j, k;
for (i = 0; i < N; i++)
{
ThisSum = 0;
for (j = i; j < N; j++)
{
ThisSum += A[j];
if (ThisSum > MaxSum)
MaxSum = ThisSum;
}
}
return MaxSum;
}
复杂度 T ( N ) = O ( N 2 ) T(N)=O(N^2) T(N)=O(N2)
int Max3(int A, int B, int C)
{ /* 返回3个整数中的最大值 */
return A > B ? A > C ? A : C : B > C ? B : C;
}
int DivideAndConquer(int List[], int left, int right)
{ /* 分治法求List[left]到List[right]的最大子列和 */
int MaxLeftSum, MaxRightSum; /* 存放左右子问题的解 */
int MaxLeftBorderSum, MaxRightBorderSum; /*存放跨分界线的结果*/
int LeftBorderSum, RightBorderSum;
int center, i;
if (left == right)
{ /* 递归的终止条件,子列只有1个数字 */
if (List[left] > 0)
return List[left];
else
return 0;
}
/* 下面是"分"的过程 */
center = (left + right) / 2; /* 找到中分点 */
/* 递归求得两边子列的最大和 */
MaxLeftSum = DivideAndConquer(List, left, center);
MaxRightSum = DivideAndConquer(List, center + 1, right);
/* 下面求跨分界线的最大子列和 */
MaxLeftBorderSum = 0;
LeftBorderSum = 0;
for (i = center; i >= left; i--)
{ /* 从中线向左扫描 */
LeftBorderSum += List[i];
if (LeftBorderSum > MaxLeftBorderSum)
MaxLeftBorderSum = LeftBorderSum;
} /* 左边扫描结束 */
MaxRightBorderSum = 0;
RightBorderSum = 0;
for (i = center + 1; i <= right; i++)
{ /* 从中线向右扫描 */
RightBorderSum += List[i];
if (RightBorderSum > MaxRightBorderSum)
MaxRightBorderSum = RightBorderSum;
} /* 右边扫描结束 */
/* 下面返回"治"的结果 */
return Max3(MaxLeftSum, MaxRightSum, MaxLeftBorderSum + MaxRightBorderSum);
}
int MaxSubseqSum3(int List[], int N)
{ /* 保持与前2种算法相同的函数接口 */
return DivideAndConquer(List, 0, N - 1);
}
代码如下图:
算法效率高是有代价的:其正确性不明显(他人难以理解算法是如何工作的)。