Solution

Method 1 – Linear Scan with Digit Sum Helper

Intuition

For each index, compute the sum of digits of nums[i] and compare to i. Return the first such index, or -1 if none exists.

Approach

Loop through the array.
For each index i, compute the sum of digits of nums[i].
If it equals i, return i. If no such index, return -1.

Code

[{"language":"C++","code":"#include <vector>\nusing namespace std;\nclass Solution {\npublic:\n    int smallestIndex(vector<int>& nums) {\n        for (int i = 0; i < nums.size(); ++i) {\n            int x = nums[i], s = 0;\n            while (x) { s += x % 10; x /= 10; }\n            if (s == i) return i;\n        }\n        return -1;\n    }\n};","lang":"cpp","html":"<pre class=\"shiki shiki-themes github-light github-dark\" style=\"background-color:#fff;--shiki-dark-bg:#24292e;color:#24292e;--shiki-dark:#e1e4e8\" tabindex=\"0\"><code><span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">#include</span><span style=\"color:#032F62;--shiki-dark:#9ECBFF\"> &#x3C;vector></span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">using</span><span style=\"color:#D73A49;--shiki-dark:#F97583\"> namespace</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\"> std</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">;</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">class</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\"> Solution</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> {</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">public:</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">    int</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\"> smallestIndex</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">(</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\">vector</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">&#x3C;</span><span style=\"color:#D73A49;--shiki-dark:#F97583\">int</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">></span><span style=\"color:#D73A49;--shiki-dark:#F97583\">&#x26;</span><span style=\"color:#E36209;--shiki-dark:#FFAB70\"> nums</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">) {</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">        for</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> (</span><span style=\"color:#D73A49;--shiki-dark:#F97583\">int</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> i </span><span style=\"color:#D73A49;--shiki-dark:#F97583\">=</span><span style=\"color:#005CC5;--shiki-dark:#79B8FF\"> 0</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">; i </span><span style=\"color:#D73A49;--shiki-dark:#F97583\">&#x3C;</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> nums.</span><span style=\"colo

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