# LeetCode

1. Two Sum

class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer, Integer> m = new HashMap<Integer, Integer>();
        int[] res = new int[2];
        for (int i = 0; i < nums.length; ++i) {
            m.put(nums[i], i);  /*第一個為值 第二個為值的位置*/
        }
        for (int i = 0; i < nums.length; ++i) {
            int t = target - nums[i]; 
            if (m.containsKey(t) && m.get(t) != i) { /*判斷t的值是否存在於map*/
                res[0] = i;
                res[1] = m.get(t);
                break;
            }
        }
        return res;
    }
}

public class Solution {
    public int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> map = new HashMap<>();
    for (int i = 0; i < nums.length; i++) {
        int complement = target - nums[i];
        if (map.containsKey(complement)) {
            return new int[] { map.get(complement), i };
        }
        map.put(nums[i], i);
    }
    throw new IllegalArgumentException("No two sum solution");
    }
}

3. Longest Substring Without Repeating Characters

class Solution {
    public int lengthOfLongestSubstring(String s) {
        int[] m = new int[256];
        Arrays.fill(m, -1); /*將陣列先填滿-1*/
        int res = 0, left = -1;
        for (int i = 0; i < s.length(); ++i) {
            left = Math.max(left, m[s.charAt(i)]);
            m[s.charAt(i)] = i;
            res = Math.max(res, i - left);
        }
        return res;
    }
}

7. Reverse Integer

class Solution {
    public int reverse(int x) {
        long ans=0;
        while(x!=0)
        {
            ans*=10;
            ans+=x%10;
            x/=10;
        }
        if((int)ans!=ans) ans=0;
        return (int)ans;

    }
}