Multiple Pointers - rohit120582sharma/Documentation GitHub Wiki

Introduction

Creating pointers or values that correspond to an index or position and move towards the beginning, end or middle based on a certain condition.

Very efficient for solving problems with minimal space complexity as well.


sumZero

Write a function called sumZero which accepts a sorted array of integers.

The function should find the first pair where the sum is 0. Return an array that includes both values that sum to zero or undefined if a pair does not exist.

Time Complexity - O(N) and Space Complexity - O(1)

function sumZero(arr){
	// Create multiple pointers
	var left = 0;
	var right = arr.length - 1;

	// Run a loop if left < right
	while(left < right){
		// Check sum
		var sum = arr[left] + arr[right];
		// If sum is '0', then return an array
		if(sum === 0){
			return [arr[left], arr[right]];
		}
		// If sum is bigger than 0, then take right down
		else if(sum > 0){
			right--;
		}
		// If sum is lesser than 0, then take left up
		else if(sum < 0){
			left++;
		}
	}
}

sumZero([-4, -3, -2, -1, 0, 1, 2, 5, 10]);
sumZero([1, 2, 3]);

countUniqueValues

Implement a function called countUniqueValues, which accepts a sorted array, and counts the unique values in the array. There can be negative numbers in the array, but it will always be sorted.

Time Complexity - O(N) and Space Complexity - O(1)

function countUniqueValues(arr){
    // Check if array is empty
    if(arr.length === 0){
        return 0;
    }
    
    // create pointer
    var i = 0;
    var n = i + 1;
    
    // Run a loop over array
    for(var j=n; j<arr.length; j++){
        if(arr[i] !== arr[j]){
            i++;
            arr[i] = arr[j];
        }
    }
    
    // return unique count
    return i + 1;
}

countUniqueValues([1,1,1,1,1,2]) // 2
countUniqueValues([1,2,3,4,4,4,7,7,12,12,13]) // 7
countUniqueValues([]) // 0
countUniqueValues([-2,-1,-1,0,1]) // 4
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