Leetcode_41_缺失的第一个正整数_思维

原地哈希,置换元素【还是标程写的简洁QAQ】

class Solution {
    boolean judge(int num, int len, int index) {
        if (num >= 1 && num <= len && num != index) {
            return true;
        }
        return false;
    }

    public int firstMissingPositive(int[] nums) {
        int len = nums.length;
        int flag = 0;
        for (int i = 0; i < len; i++) {
            if (judge(nums[i], len, i +1)) {
                if (judge(nums[nums[i] - 1], len, nums[i])) {
                    flag = nums[nums[i] - 1];
                }
                nums[nums[i] - 1] = nums[i];
                while (flag != 0) {
                    int temp = nums[flag - 1];
                    nums[flag - 1] = flag;
                    if (judge(temp, len, flag)) {
                        flag = temp;
                    } else {
                        flag = 0;
                    }
                }
            }
        }
        for (int i = 0; i < len; i++) {
            if (nums[i] != i + 1) {
                return i + 1;
            }
        }
        return len + 1;
    }
}

标程

class Solution {
    public int firstMissingPositive(int[] nums) {
        int n = nums.length;
        for (int i = 0; i < n; ++i) {
            while (nums[i] > 0 && nums[i] <= n && nums[nums[i] - 1] != nums[i]) {
                int temp = nums[nums[i] - 1];
                nums[nums[i] - 1] = nums[i];
                nums[i] = temp;
            }
        }
        for (int i = 0; i < n; ++i) {
            if (nums[i] != i + 1) {
                return i + 1;
            }
        }
        return n + 1;
    }
}

2022/3/10用标记法完成

class Solution {
    public int firstMissingPositive(int[] nums) {
        int n = nums.length;
        for(int i = 0;i < n;i++) {
            if(nums[i] <= 0) {
                nums[i] = n + 1;
            }
        }
        for(int i = 0;i < n;i++) {
            int temp = Math.abs(nums[i]);
            if(temp <= n && temp >= 1 && nums[temp - 1] > 0) {
                nums[temp - 1] = -nums[temp - 1];
            }
        }
        for(int i = 0;i < n;i++) {
            if(nums[i] > 0) {
                return i + 1;
            }
        }
        return n + 1;
    }
}