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217. There are duplicate elements

Title Description

 Given an array of integers , Determine whether there are duplicate elements .
 If a value exists, it appears at least twice in the array , The function returns  true . If every element in the array is different , Then return to  false .

Code implementation

Method 1 double for

const nums = [1, 2, 3, 1];
const nums2 = [1, 2, 3];
//  Still shake hands 
var containsDuplicate = function(nums) {
    
    for (let i = 0; i < nums.length; i++) {
    
        for (let j = i + 1; j < nums.length; j++) {
    
            if (nums[i] == nums[j]) {
    
                return true
            }
        }
    }
    return false
};
console.log(containsDuplicate(nums));
console.log(containsDuplicate(nums2));

Method 2Set+size

const nums = [1, 2, 3, 1];
const nums2 = [1, 2, 3];
var containsDuplicate = function(nums) {
    
    // jsSet It has a de duplication effect 
    return new Set(nums).size == nums.length;
}; 
console.log(containsDuplicate(nums));
console.log(containsDuplicate(nums2));

Method 3Object+keys()+length

const nums = [1, 2, 3, 1];
const nums2 = [1, 2, 3];
var containsDuplicate = function(nums) {
    
    // js Objects have unique properties 
    let obj = {
    };
    for (const iterator of nums) {
    
        obj[iterator] = iterator;
    }
    return Object.keys(obj).length == nums.length;
};
console.log(containsDuplicate(nums));
console.log(containsDuplicate(nums2));

Method 4forEach()+indexof()+length

const nums = [1, 2, 3, 1];
const nums2 = [1, 2, 3];
var containsDuplicate = function(nums) {
    
   //  Traverse to determine whether there are values in the new array , Do not add values repeatedly , Finally, judge whether the length of the new array is equal to that of the array 
   let arr = [];
   nums.forEach(function(item) {
    
       if (arr.indexOf(item) == -1) {
    
           arr.push(item);
       }
   })
   return arr.length == nums.length;
}
console.log(containsDuplicate(nums));
console.log(containsDuplicate(nums2));

Method 5sort()+for

const nums = [1, 2, 3, 1];
const nums2 = [1, 2, 3];
var containsDuplicate = function(nums) {
    
    //  Prioritize , Then judge whether the adjacent items are equal 
    nums.sort();
    for (let i = 0; i < nums.length - 1; i++) {
    
        if (nums[i] == nums[i + 1]) {
    
            return true;
        }
    }
    return false;
};
console.log(containsDuplicate(nums));
console.log(containsDuplicate(nums2));