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add binary search algo

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This commit is contained in:
ehlxr 2022-01-02 00:25:11 +08:00
parent 0f081ad1db
commit beb3a22fdd
1 changed files with 93 additions and 13 deletions
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106
budd-common/src/main/java/io/github/ehlxr/algorithm/search/BinarySearch.java
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@ -35,6 +35,12 @@ public class BinarySearch {
System.out.println(searchRec(a, a.length, 3)); System.out.println(searchRec(a, a.length, 3));
a = new int[]{1, 2, 2, 2, 2, 5, 6, 7, 8, 9}; a = new int[]{1, 2, 2, 2, 2, 5, 6, 7, 8, 9};
System.out.println(searchFirst(a, a.length, 2)); System.out.println(searchFirst(a, a.length, 2));
a = new int[]{1, 3, 4, 5, 6, 8, 8, 8, 9};
System.out.println(searchLast(a, a.length, 8));
a = new int[]{1, 2, 2, 3, 4, 5, 6, 7, 8, 8, 9};
System.out.println(searchFirstGreater(a, a.length, 7));
a = new int[]{1, 2, 3, 4, 5, 6, 8, 9};
System.out.println(searchLastLess(a, a.length, 7));
} }
/** /**
@ -49,12 +55,10 @@ public class BinarySearch {
int low = 0; int low = 0;
int high = n - 1; int high = n - 1;
while (low <= high) { while (low <= high) {
int mid = low + (high - low) >> 1; int mid = low + ((high - low) >> 1);
if (a[mid] == v) { if (a[mid] == v) {
return mid; return mid;
} } else if (a[mid] < v) {
if (a[mid] < v) {
low = mid + 1; low = mid + 1;
} else { } else {
high = mid - 1; high = mid - 1;
@ -83,12 +87,10 @@ public class BinarySearch {
return -1; return -1;
} }
int mid = low + (high - low) >> 1; int mid = low + ((high - low) >> 1);
if (a[mid] == v) { if (a[mid] == v) {
return mid; return mid;
} } else if (a[mid] < v) {
if (a[mid] < v) {
return searchRec(a, mid + 1, high, v); return searchRec(a, mid + 1, high, v);
} else { } else {
return searchRec(a, low, mid - 1, v); return searchRec(a, low, mid - 1, v);
@ -107,16 +109,14 @@ public class BinarySearch {
int low = 0; int low = 0;
int high = n - 1; int high = n - 1;
while (low <= high) { while (low <= high) {
int mid = low + (high - low) >> 1; int mid = low + ((high - low) >> 1);
if (a[mid] == v) { if (a[mid] == v) {
if (a[mid - 1] != v) { if (mid == 0 || a[mid - 1] != v) {
return mid; return mid;
} else { } else {
high = mid - 1; high = mid - 1;
} }
} } else if (a[mid] < v) {
if (a[mid] < v) {
low = mid + 1; low = mid + 1;
} else { } else {
high = mid - 1; high = mid - 1;
@ -124,4 +124,84 @@ public class BinarySearch {
} }
return -1; return -1;
} }
/**
* 查找最后一个值等于给定值的元素
*
* @param a 要查找的数组
* @param n 数组的长度
* @param v 要查找的值
* @return 要查找值在数组中的索引
*/
public static int searchLast(int[] a, int n, int v) {
int low = 0;
int high = n - 1;
while (low <= high) {
int mid = low + ((high - low) >> 1);
if (a[mid] == v) {
if (mid == n - 1 || a[mid + 1] != v) {
return mid;
} else {
low = mid + 1;
}
} else if (a[mid] < v) {
low = mid + 1;
} else {
high = mid - 1;
}
}
return -1;
}
/**
* 查找第一个大于等于给定值的元素
*
* @param a 要查找的数组
* @param n 数组的长度
* @param v 要查找的值
* @return 要查找值在数组中的索引
*/
public static int searchFirstGreater(int[] a, int n, int v) {
int low = 0;
int high = n - 1;
while (low <= high) {
int mid = low + ((high - low) >> 1);
if (a[mid] < v) {
low = mid + 1;
} else {
if (mid == 0 || a[mid - 1] < v) {
return mid;
} else {
high = mid - 1;
}
}
}
return -1;
}
/**
* 查找最后一个小于等于给定值的元素
*
* @param a 要查找的数组
* @param n 数组的长度
* @param v 要查找的值
* @return 要查找值在数组中的索引
*/
public static int searchLastLess(int[] a, int n, int v) {
int low = 0;
int high = n - 1;
while (low <= high) {
int mid = low + ((high - low) >> 1);
if (a[mid] > v) {
high = mid - 1;
} else {
if (mid == n - 1 || a[mid + 1] > v) {
return mid;
} else {
low = mid + 1;
}
}
}
return -1;
}
} }