leetcode java hashMap

HashMap常用方法

• add element: map.put(key, value)
• from key get value: map.get(key)
• Build HashMap: getOrDefault
• iterate HashMap: for(? i:map.keySet())
• String HashMap

1512. Number of Good Pairs

leetcode.com/problems/nu…

Given an array of integers nums.

A pair (i,j) is called good if nums[i] == nums[j] and i < j.

Return the number of good pairs.

Example 1:

Input: nums = [1,2,3,1,1,3] Output: 4 Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5) 0-indexed.

class Solution {
    public int numIdenticalPairs(int[] nums) {
        
        int res = 0;
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        //calculate count for each key
        for(int x : nums){
            map.put(x, map.getOrDefault(x, 0)+1);            
        }
        
        //iterate
        for(Integer i : map.keySet()){
            System.out.println("key: " + i + "value: " + map.get(i));
        }
        
        
        for(Integer i : map.keySet()){
            if(map.get(i) > 1){
                int n = map.get(i);
                for(int j = 1; j <= n-1; j++){
                res += j;
                }
            }
        }
        
        return res;
    }
}

• Build HashMap: getOrDefault
• iterate HashMap: for(? i:map.keySet())

771. Jewels and Stones

You're given strings jewels representing the types of stones that are jewels, and stones representing the stones you have. Each character in stones is a type of stone you have. You want to know how many of the stones you have are also jewels.

Letters are case sensitive, so "a" is considered a different type of stone from "A".

Example 1:

Input: jewels = "aA", stones = "aAAbbbb" Output: 3

class Solution {
    public int numJewelsInStones(String jewels, String stones) {
        
        int res = 0;
        HashMap<Character, Integer> map = new HashMap<Character, Integer>();
        for(char x : jewels.toCharArray()){
            map.put(x, 0);
        }
        
        for(char x: stones.toCharArray()){
            if(map.containsKey(x)){
                map.put(x, map.getOrDefault(x, 0)+1);
                res += 1;  
            }

        }
        return res;
    }
}

• String HashMap

1. Two Sum

leetcode.com/problems/tw…

Example 1:

Input: nums = [2,7,11,15], target = 9 Output: [0,1] Output: Because nums[0] + nums[1] == 9, we return [0, 1].

class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
		int[] defaultResult = {0, 0};
		for (int i = 0; i < nums.length; i++) {
			if (map.get(target-nums[i]) != null ) {
				int[] result = {map.get(target-nums[i]), i };
				return result;
			}
			map.put(nums[i], i);
		}
		return defaultResult;
    }
}

• how to prevent duplicate e.g. nums=[3, 3], target = 6, Output = [0, 1]