LeetCode - Easy - 27. Remove Element

末蓝、 2022-12-27 07:24 133阅读 0赞

## Topic ##

Array, Two Pointers

## Description ##

[https://leetcode.com/problems/remove-element/][https_leetcode.com_problems_remove-element]

Given an array nums and a value val, remove all instances of that value in-place and return the new length.

Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.

The order of elements can be changed. It doesn’t matter what you leave beyond the new length.

--------------------

**Clarification**:

Confused why the returned value is an integer but your answer is an array?

Note that the input array is passed in by reference, which means a modification to the input array will be known to the caller as well.

Internally you can think of this:

// nums is passed in by reference. (i.e., without making a copy)
    int len = removeElement(nums, val);
    
    // any modification to nums in your function would be known by the caller.
    // using the length returned by your function, it prints the first len elements.
    for (int i = 0; i < len; i++) { 
        print(nums[i]);
    }

--------------------

**Example 1**:

Input: nums = [3,2,2,3], val = 3
    Output: 2, nums = [2,2]
    Explanation: Your function should return length = 2, with the first two elements of nums being 2.
    It doesn't matter what you leave beyond the returned length. For example if you return 2 with nums = [2,2,3,3] or nums = [2,3,0,0], your answer will be accepted.

**Example 2**:

Input: nums = [0,1,2,2,3,0,4,2], val = 2
    Output: 5, nums = [0,1,4,0,3]
    Explanation: Your function should return length = 5, with the first five elements of nums containing 0, 1, 3, 0, and 4. Note that the order of those five elements can be arbitrary. It doesn't matter what values are set beyond the returned length.

**Constraints**:

*  0 <= nums.length <= 100
 *  0 <= nums\[i\] <= 50
 *  0 <= val <= 100

## Analysis ##

方法一：遍历数组，遇到待删值，则从数组尾调取元素，取代待删值。如果待删值与尾元素相等，则取尾元素的前一元素，直到不相等为止。

方法二（推荐）：遍历数组，若遇到待删值，则忽略。若遇到非待删值，则将其赋值到数组前部。

## Submission ##

public class RemoveElement { 
    	//方法1
    	public int removeElement(int[] nums, int val) { 
    		int lastIndex = 0;
    
    		if (nums.length > 0) { 
    
    			lastIndex = nums.length - 1;
    
    			for (int i = 0; i <= lastIndex; i++) { 
    				while (nums[i] == val) { 
    					nums[i] = nums[lastIndex--];
    					if (i > lastIndex)
    						break;
    				}
    			}
    			lastIndex++;
    		}
    
    		return lastIndex;
    	}
    
    	//方法2
    	public int removeElement2(int[] nums, int val) { 
    		int m = 0;
    		for (int i = 0; i < nums.length; i++) { 
    
    			if (nums[i] != val) { 
    				nums[m] = nums[i];
    				m++;
    			}
    		}
    
    		return m;
    	}
    }

## Test ##

import static org.junit.Assert.*;
    
    import java.util.Arrays;
    
    import org.junit.Test;
    
    public class RemoveElementTest { 
    
    	@Test
    	public void test() { 
    		RemoveElement obj = new RemoveElement();
    
    		
    		int[] array1 = { 3, 2, 2, 3};
    		int expectedNum1 = 2;
    		assertEquals(expectedNum1, obj.removeElement(array1, 3));
    		assertArrayEquals(new int[] { 2, 2}, Arrays.copyOf(array1, expectedNum1));
    		
    		
    		int[] array2 = { 0, 1, 2, 2, 3, 0, 4, 2};
    		int expectedNum2 = 5;
    		assertEquals(expectedNum2, obj.removeElement(array2, 2));
    		assertArrayEquals(new int[] { 0, 1, 4, 0, 3}, Arrays.copyOf(array2, expectedNum2));
    		
    		//---
    		int[] array3 = { 1};
    		int expectedNum3 = 1;
    		assertEquals(expectedNum3, obj.removeElement(array3, 2));
    		assertArrayEquals(new int[] { 1}, Arrays.copyOf(array3, expectedNum3));
    		
    		int[] array4 = { 1};
    		int expectedNum4 = 0;
    		assertEquals(expectedNum4, obj.removeElement(array4, 1));
    		assertArrayEquals(new int[] { }, Arrays.copyOf(array4, expectedNum4));
    		
    		
    		int[] array5 = { 4, 5};
    		int expectedNum5 = 1;
    		assertEquals(expectedNum5, obj.removeElement(array5, 5));
    		assertArrayEquals(new int[] { 4}, Arrays.copyOf(array5, expectedNum5));
    
    	}
    	
    	
    	@Test
    	public void test2() { 
    		RemoveElement obj = new RemoveElement();
    		
    		int[] array1 = { 3, 2, 2, 3};
    		int expectedNum1 = 2;
    		assertEquals(expectedNum1, obj.removeElement2(array1, 3));
    		assertArrayEquals(new int[] { 2, 2}, Arrays.copyOf(array1, expectedNum1));
    		
    		//---
    		int[] array3 = { 1};
    		int expectedNum3 = 1;
    		assertEquals(expectedNum3, obj.removeElement2(array3, 2));
    		assertArrayEquals(new int[] { 1}, Arrays.copyOf(array3, expectedNum3));
    		
    		int[] array4 = { 1};
    		int expectedNum4 = 0;
    		assertEquals(expectedNum4, obj.removeElement2(array4, 1));
    		assertArrayEquals(new int[] { }, Arrays.copyOf(array4, expectedNum4));
    		
    		
    		int[] array5 = { 4, 5};
    		int expectedNum5 = 1;
    		assertEquals(expectedNum5, obj.removeElement2(array5, 5));
    		assertArrayEquals(new int[] { 4}, Arrays.copyOf(array5, expectedNum5));
    		
    	}
    	
    	
    }

[https_leetcode.com_problems_remove-element]: https://leetcode.com/problems/remove-element/